Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Mar. 09, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	Aclaris Therapeutics, Inc.
Entity Central Index Key	0001557746
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		30,901,492
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Entity Public Float			$ 639.2

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 20,202	$ 30,171
Marketable securities	173,655	107,051
Accounts receivable, net	481
Prepaid expenses and other current assets	5,883	1,334
Total current assets	200,221	138,556
Marketable securities	14,997	36,912
Property and equipment, net	2,159	481
Intangible assets	7,349
Goodwill	18,504
Other assets	279	136
Total assets	243,509	176,085
Current liabilities:
Accounts payable	7,822	2,845
Accrued expenses	4,940	3,378
Total current liabilities	12,762	6,223
Contingent consideration	4,378
Other liabilities	558	372
Deferred tax liability	549
Total liabilities	18,247	6,595
Stockholders' Equity:
Preferred stock, $0.0001 par value; 10,000,000 shares authorized and no shares issued or outstanding at December 31, 2017 and December 31, 2016
Common stock, $0.00001 par value; 100,000,000 shares authorized at December 31, 2017 and December 31, 2016; 30,834,679 and 26,059,181 shares issued and outstanding at December 31, 2017 and December 31, 2016, respectively
Additional paid-in capital	384,943	260,671
Accumulated other comprehensive loss	(246)	(269)
Accumulated deficit	(159,435)	(90,912)
Total stockholders’ equity	225,262	169,490
Total liabilities and stockholders’ equity	$ 243,509	$ 176,085

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Consolidated Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.00001	$ 0.00001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	30,856,505	26,059,181
Common stock, shares outstanding	30,834,679	26,059,181

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements Of Operations And Comprehensive Loss
Revenue	$ 1,683
Cost of revenue	1,207
Gross profit	476
Operating expenses:
Research and development	39,790	$ 33,476	$ 15,339
General and administrative	33,109	15,091	5,328
Total operating expenses	72,899	48,567	20,667
Loss from operations	(72,423)	(48,567)	(20,667)
Other income, net	2,070	488	104
Loss before income taxes	(70,353)	(48,079)	(20,563)
Provision for (benefit from) income taxes	(1,830)
Net loss	(68,523)	(48,079)	(20,563)
Accretion of convertible preferred stock			(2,566)
Net loss attributable to common stockholders	$ (68,523)	$ (48,079)	$ (23,129)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (2.44)	$ (2.25)	$ (3.79)
Weighted average common shares outstanding, basic and diluted (in shares)	28,102,386	21,415,733	6,107,042
Other comprehensive loss:
Unrealized (loss) gain on marketable securities, net of tax of $0	$ (121)	$ 105	$ (148)
Foreign currency translation adjustments	144	(225)	5
Total other comprehensive income (loss)	23	(120)	(143)
Comprehensive loss	$ (68,500)	$ (48,199)	$ (20,706)

CONSOLIDATED STATEMENT OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock Private Placement	Common Stock Follow On Public Offering	Common Stock At The Market Offering [Member]	Common Stock	Additional Paid-In Capital Private Placement	Additional Paid-In Capital Follow On Public Offering	Additional Paid-In Capital At The Market Offering [Member]	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit	Private Placement	Follow On Public Offering	At The Market Offering [Member]	Total
Balance at Dec. 31, 2014														$ 36,677
Balance (in shares) at Dec. 31, 2014														27,341,057
Increase (Decrease) in Temporary Equity
Issuance of convertible preferred stock, net of issuance costs														$ 39,864
Issuance of convertible preferred stock (in shares)														12,944,984
Issuance of common stock upon conversion of convertible preferred stock														$ (78,305)
Conversion of convertible preferred stock into permanent equity (in shares)														(40,286,041)
Accretion of convertible preferred stock to redemption value														$ 1,764
Balance at Dec. 31, 2014									$ (6)	$ (20,749)				(20,755)
Balance (in shares) at Dec. 31, 2014				2,730,427
Increase (Decrease) in Stockholders' Equity
Issuance of common stock								$ 56,550						56,550
Number of shares issued				5,750,000
Issuance of stock upon conversion								78,305						78,305
Issuance of stock upon conversion, shares				11,677,076
Unrealized loss on marketable securities									(148)					(148)
Foreign currency translation adjustment									5					5
Stock-based compensation expense								891						891
Accretion of convertible preferred stock, APIC effect								(243)
Accretion of convertible preferred stock, equity effect										(1,521)				(1,764)
Operating loss										(20,563)				(20,563)
Balance at Dec. 31, 2015								135,503	(149)	(42,833)				92,521
Balance (in shares) at Dec. 31, 2015				20,157,503
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with acquisition								2,355						2,355
Issuance of common stock in connection with acquisition, shares				159,420
Issuance of common stock					$ 18,547	$ 98,158					$ 18,547	$ 98,158
Number of shares issued	1,081,082	4,600,000
Exercise of stock options and vesting of RSUs								4						4
Exercise of stock options and vesting of RSUs, shares				61,176
Unrealized loss on marketable securities									105					105
Foreign currency translation adjustment									(225)					(225)
Stock-based compensation expense								6,104						6,104
Operating loss										(48,079)				(48,079)
Balance at Dec. 31, 2016								260,671	(269)	(90,912)				$ 169,490
Balance (in shares) at Dec. 31, 2016				26,059,181										26,059,181
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in connection with acquisition								9,675						$ 9,675
Issuance of common stock in connection with acquisition, shares				349,527
Issuance of common stock						$ 80,918	$ 19,311					$ 80,918	$ 19,311
Number of shares issued		3,747,602	635,000										635,000
Exercise of stock options and vesting of RSUs								(62)						(62)
Exercise of stock options and vesting of RSUs, shares				65,195
Unrealized loss on marketable securities									(121)					(121)
Foreign currency translation adjustment									144					144
Stock-based compensation expense								14,430						14,430
Operating loss										(68,523)				(68,523)
Balance at Dec. 31, 2017								$ 384,943	$ (246)	$ (159,435)				$ 225,262
Balance (in shares) at Dec. 31, 2017				30,856,505										30,856,505

CONSOLIDATED STATEMENT OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Offering costs netted against proceeds	$ 5,352
Temporary equity, issuance costs			$ 136
IPO
Offering costs netted against proceeds			$ 2,272
Private Placement
Offering costs netted against proceeds		$ 1,453
Follow On Public Offering
Offering costs netted against proceeds		$ 6,492
At The Market Offering [Member]
Offering costs netted against proceeds	$ 691

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (68,523)	$ (48,079)	$ (20,563)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	402	120	90
Stock-based compensation expense	14,430	6,104	891
Deferred taxes	(1,837)
Write-down of property and equipment held for sale		216	289
Non-cash charges related to Vixen acquisition		2,784
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(4,306)	(8)	(1,269)
Accounts payable	4,564	1,810	(359)
Accrued expenses	607	2,450	552
Net cash used in operating activities	(54,663)	(34,603)	(20,369)
Cash flows from investing activities:
Purchases of property and equipment	(1,235)	(232)	(507)
Acquisition of Confluence, net of cash acquired	(9,647)
Purchases of marketable securities	(197,337)	(148,764)	(82,513)
Proceeds from sales and maturities of marketable securities	152,527	87,093	6,069
Net cash used in investing activities	(55,692)	(61,903)	(76,951)
Cash flows from financing activities
Proceeds from sale of stock, net of issuance costs	80,918	98,158
Proceeds from issuance of convertible preferred stock, net of issuance costs			39,864
Capital lease payments	(78)
Proceeds from the exercise of employee stock options	235	121
Net cash provided by financing activities	100,386	116,826	96,414
Net (decrease) increase in cash and cash equivalents	(9,969)	20,320	(906)
Cash and cash equivalents at beginning of period	30,171	9,851	10,757
Cash and cash equivalents at end of period	20,202	30,171	9,851
Supplemental disclosures of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable	274	11	2
Fair value of stock issued in connection with acquisition	9,675	2,355
Offering costs included in accounts payable	20	250
Accretion of convertible preferred stock to redemption value			1,764
Private Placement
Cash flows from financing activities
Proceeds from sale of stock, net of issuance costs		$ 18,547
IPO
Cash flows from financing activities
Proceeds from sale of stock, net of issuance costs			$ 56,550
At The Market Offering [Member]
Cash flows from financing activities
Proceeds from sale of stock, net of issuance costs	$ 19,311

Organization and Nature of Business	12 Months Ended
Organization and Nature of Business	Dec. 31, 2017
Organization and Nature of Business
Organization and Nature of Business	1. Organization and Nature of Business Overview Aclaris Therapeutics, Inc. was incorporated under the laws of the State of Delaware in 2012. In July 2015, Aclaris Therapeutics International Limited (“ATIL”) was established under the laws of the United Kingdom as a wholly-owned subsidiary of Aclaris Therapeutics, Inc. In March 2016, Vixen Pharmaceuticals, Inc. (“Vixen”) became a wholly-owned subsidiary of Aclaris Therapeutics, Inc. (see Note 12). In August 2017, Confluence Life Sciences Inc. (“Confluence”) was acquired by Aclaris Therapeutics, Inc. and became a wholly-owned subsidiary thereof (see Note 3). Aclaris Therapeutics, Inc., ATIL, Vixen and Confluence are referred to collectively as the “Company”. The Company is a dermatologist-led biopharmaceutical company focused on identifying, developing and commercializing innovative and differentiated therapies to address significant unmet needs in medical and aesthetic dermatology. The Company’s lead drug, ESKATA (Hydrogen Peroxide) Topical Solution, 40% (w/w) (“ESKATA”), is a proprietary high‑concentration formulation of hydrogen peroxide that the Company is commercializing as a prescription treatment for raised seborrheic keratosis (“SK”), a common non‑malignant skin tumor. In February 2017, the Company submitted a New Drug Application (“NDA”) for ESKATA to the U.S. Food and Drug Administration (“FDA”). The NDA was approved by the FDA in December 2017. Liquidity The Company’s consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. At December 31, 2017, the Company had cash, cash equivalents and marketable securities of $208,854 and an accumulated deficit of $159,435. Since inception, the Company has incurred net losses and negative cash flows from its operations. Prior to the acquisition of Confluence in August 2017, the Company had never generated any revenue. There is no assurance that profitable operations will ever be achieved, and, if achieved, will be sustained on a continuing basis. In addition, development activities, clinical and pre-clinical testing, and commercialization of the Company’s drug candidates will require significant additional financing. The Company expects that its cash, cash equivalents and marketable securities as of December 31, 2017 will be sufficient to fund its operations for a period greater than 12 months from the date of issuance of these consolidated financial statements based on its current operating assumptions. The future viability of the Company is dependent on its ability to generate cash from operating activities or to raise additional capital to finance its operations. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and ability to pursue its business strategies.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). The financial statements include the consolidated accounts of the Company and its wholly-owned subsidiaries, Confluence, ATIL and Vixen. All significant intercompany transactions have been eliminated.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, research and development expenses, contingent consideration and the valuation of stock-based awards.

Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Revenue Recognition

The Company recognizes revenue when the earnings process is complete, which under SEC Staff Accounting

Bulletin No. 104, Topic No. 13, “Revenue Recognition,” is when revenue is realized or realizable and earned, there is persuasive evidence a revenue arrangement exists, delivery of goods or services has occurred, the sales price is fixed or determinable, and collectability is reasonably assured.

The Company earns revenue from the provision of laboratory services to clients through Confluence, its wholly-owned subsidiary. Laboratory service revenue is generally evidenced by contracts with clients which are on an agreed upon fixed-price, fee-for-service basis and are generally billed on a monthly basis in arrears for services rendered. Revenue related to these contracts is generally recognized as the laboratory services are performed, based upon the rates specified in the contracts.

The Company also receives revenue from grants under the Small Business Innovation Research program of the National Institutes of Health (“NIH”). The Company, through its Confluence subsidiary, currently has two active grants from NIH which are related to early-stage research. The Company recognizes revenue related to these grants as amounts become reimbursable under each grant, which is generally when research is performed and the related costs are incurred.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development expenses include salaries, stock-based compensation and benefits of employees, fees paid under licensing agreements, fees paid under a third party assignment agreement and other operational costs related to the Company’s research and development activities, including depreciation expenses and the cost of research and development contracts which the Company has entered into with outside vendors to conduct both preclinical studies and clinical trials. Significant judgment and estimates are made in determining the amount of research and development costs recognized in each reporting period. The Company analyzes the progress of its preclinical studies and clinical trials, completion of milestone events, invoices received and contracted costs when estimating research and development costs. Actual results could differ from the Company’s estimates. The Company’s historical estimates for research and development costs have not been materially different from the actual costs.

Foreign Currency Translation

The reporting currency of the Company is the U.S. Dollar. The functional currency of ATIL, the Company’s wholly-owned subsidiary, is the British Pound. Assets and liabilities of ATIL are translated into U.S. Dollars based on exchange rates at the end of each reporting period. Revenues and expenses are translated at average exchange rates during the reporting period. Gains and losses arising from the translation of assets and liabilities are included as a component of accumulated other comprehensive loss within the Company’s consolidated balance sheet. Gains and losses resulting from foreign currency transactions are reflected within the Company’s consolidated statements of operations. The Company has not utilized foreign currency hedging strategies to mitigate the effect of its foreign currency exposure.

Stock-Based Compensation

The Company measures the compensation expense of stock-based awards granted to employees and directors using the grant date fair value of the award. The Company has issued stock options and restricted stock unit (“RSU”) awards with service-based vesting conditions, as well as with performance-based vesting conditions. The Company has not issued awards that include market-based conditions. For service-based awards the Company recognizes stock-based compensation expense on a straight-line basis over the requisite service period. For performance-based awards the Company recognizes stock-based compensation expense on a straight-line basis over the requisite service period beginning in the period that it becomes probable the performance conditions will occur. At each balance sheet date, the Company evaluates whether any performance conditions related to a performance-based award have changed. The effect of any change in performance conditions would be recognized as a cumulative catch-up adjustment in the period such change occurs, and any remaining unrecognized compensation expense would be recognized on a straight-line basis over the remaining requisite service period. The impact of forfeitures is recognized in the period in which they occur.

The Company initially measures the compensation expense of stock-based awards granted to consultants using the grant date fair value of the award. Compensation expense is recognized over the period during which services are rendered by such consultants. At the end of each financial reporting period prior to completion of services being rendered, the compensation expense related to these awards is remeasured using the then current fair value of the Company’s common stock for RSUs, or based upon updated assumptions in the Black-Scholes option pricing model for stock option awards.

The Company classifies stock-based compensation expense in its statement of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified.

The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company estimates its expected stock volatility based on the historical volatility of a set of peer companies, which are publicly traded, and expects to continue to do so until it has adequate historical data regarding the volatility of its own publicly-traded stock price. The expected term of the Company’s stock options has been determined using the “simplified” method for awards that qualify as “plain vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The Company uses an expected dividend yield of zero based on the fact that the Company has never paid cash dividends and does not expect to pay cash dividends in the future. Prior to the IPO, the Company valued its common stock using a hybrid method to estimate its enterprise value. The hybrid method used was a probability-weighted expected return method which was a scenario-based methodology that estimated the fair value of the Company’s common stock based upon an analysis of future values for the Company assuming various outcomes. The hybrid method used calculated equity values using an option pricing model in one or more of scenarios, and also considered the rights of each class of stock.

The fair value of each RSU is measured using the closing price of the Company’s common stock on the date of grant.

Patent Costs

All patent related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as general and administrative expenses.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial statement and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes, based upon the weight of available evidence, that it is more likely than not that all or a portion of the deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more likely than not to be sustained, the tax position is then assessed to determine the amount of benefit to recognize in the consolidated financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Reverse Stock Split

In September 2015, the Company effected a 1‑for‑3.45 reverse stock split of its issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each series of the Company’s then-outstanding convertible preferred stock. Accordingly, all share and per share amounts for all periods presented in these consolidated financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect this reverse stock split and adjustment of the preferred stock conversion ratios.

Accretion of Convertible Preferred Stock

Accretion of convertible preferred stock included the accretion of accruing dividends on and issuance costs of the Company’s Series A, B and C convertible preferred stock. The carrying values of the Series A and Series B convertible preferred stock were accreted to their respective redemption values, using the effective interest method, from the date of issuance through August 28, 2015. In connection with the closing of the Company’s Series C convertible preferred stock financing on August 28, 2015, the redemption rights of the Series A and B convertible preferred stock were removed. Subsequent to August 28, 2015, the Company was no longer required to record the accumulated undeclared dividends on its balance sheet, but was thereafter required to deduct accumulated undeclared dividends as part of its earnings per share calculation. In October 2015, in connection with the Company’s IPO, all of the Company’s convertible preferred stock was converted to common stock.

Comprehensive Loss

Comprehensive loss includes net loss as well as other changes in stockholders’ equity (deficit) that result from transactions and economic events other than those with stockholders. Comprehensive loss is comprised of net loss, foreign currency translation adjustments and unrealized gains (losses) on marketable securities.

Net Loss per Share

Basic net loss per share is computed using the weighted average number of common shares outstanding during the period. Diluted net loss per share is computed using the sum of the weighted average number of common shares outstanding during the period, plus the weighted average number of potential shares of common stock from the assumed exercise of stock options, and the assumed vesting of RSUs and restricted stock granted by the Company upon its formation, if dilutive. Prior to the IPO, the Company applied the two-class method of calculating its basic and diluted net loss per share attributable to common stockholders since its convertible preferred stock and common stock were participating securities. The Company’s convertible preferred stock contractually entitled the holders of such shares to participate in dividends, but not in losses, of the Company. Since the Company was in a net loss position, and preferred stockholders did not participate in losses, basic and diluted net loss per share was the same for each of the periods presented.

Cash Equivalents

The Company considers all short term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents. Cash equivalents, which have consisted of money market accounts, commercial paper and corporate debt securities with original maturities of less than three months, are stated at fair value.

Marketable Securities

Marketable securities with original maturities of greater than three months and remaining maturities of less than one year from the balance sheet date are classified as short term. Marketable securities with remaining maturities of greater than one year from the balance sheet date are classified as long term.

The Company classifies all of its marketable securities as available-for-sale securities. The Company’s marketable securities are measured and reported at fair value using quoted prices in markets that are not active for identical or similar securities. Unrealized gains and losses are reported as a separate component of stockholders’ equity. The cost of securities sold is determined on a specific identification basis, and realized gains and losses, if any, are included in other income, net within the consolidated statement of operations and comprehensive loss. If any adjustment to fair value reflects a decline in the value of the investment, the Company considers available evidence to evaluate the extent to which the decline is “other than temporary” and reduces the investment to fair value through a charge to the statement of operations and comprehensive loss.

Assets Held for Sale

In order for an asset to be classified as held for sale, several criteria must be achieved. These criteria include, among others, an active program to market an asset and locate a buyer, as well as the probable disposition of the asset within one year. Upon being classified as held for sale, the recoverability of the carrying value of an asset must be assessed and evaluated. After the valuation process is completed, the held for sale asset is reported at the lower of its carrying value or fair value less cost to sell, and no additional depreciation expense is recognized related to the asset. Once an asset is classified as held for sale, all of its historical balance sheet information is included in prepaid expenses and other current assets in the accompanying consolidated balance sheets. During the year ended December 31, 2015, the Company determined that several pieces of machinery used in the Company’s scale-up operations would no longer be part of the Company’s future operations. The Company engaged a third-party to market the assets and locate a buyer in the fourth quarter of 2015. During the year ended December 31, 2016, the Company was unable to locate a buyer for the machinery and, therefore, wrote the remaining value of the machinery down to zero. The Company recorded impairment charges of $216 and $289 in the years ended December 31, 2016 and 2015, respectively. The impairment charges are included in research and development expense on the Company’s consolidated statement of operations and comprehensive loss. The Company had no assets classified as held for sale as of December 31, 2017 and 2016.

Other Assets

In February 2017, the Company paid a $2.0 million PDUFA fee to the FDA in conjunction with the filing of its NDA for ESKATA. The Company requested a waiver and refund of this PDUFA fee, which was approved by the FDA in December 2017. The amount paid by the Company has been recorded in prepaid expenses and other current assets on the Company’s consolidated balance sheet since the refund was not received until after December 31, 2017.

Fair Value Measurements

Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Financial assets and liabilities carried at fair value are to be classified and disclosed in one of the following three levels of the fair value hierarchy, of which the first two are considered observable and the last is considered unobservable:

Level 1 — Quoted prices in active markets for identical assets or liabilities.

Level 2 — Observable inputs (other than Level 1 quoted prices), such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data.

Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques.

The Company’s cash equivalents, marketable securities and contingent consideration are carried at fair value, determined according to the fair value hierarchy described above. The carrying value of the Company’s accounts payable and accrued expenses approximate fair value due to the short-term nature of these liabilities.

Concentration of Credit Risk and of Significant Customers and Suppliers

Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents and marketable securities. The Company holds all cash, cash equivalents and marketable securities balances at one accredited financial institution, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

The Company’s top five customers represented 70% of total laboratory service revenues earned from August 3, 2017, the date of acquisition of Confluence, through December 31, 2017.

The Company is dependent on third party manufacturers to supply products for research and development activities of its programs, including preclinical and clinical testing. These programs could be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients and other components.

Deferred Offering Costs

The Company recorded legal, accounting and other third-party fees associated directly with the filing of its registration statement on Form S-3 in November 2016, in other assets on its consolidated balance sheet. These deferred offering costs are recorded in stockholders’ equity as a reduction of the proceeds generated from offerings consummated under the Form S-3 on a pro rata basis. The Company may also record legal, accounting and other third-party fees directly associated with in-process equity financings as deferred offering costs (non-current) until such financings are completed. The deferred costs related to an in-process equity financing are recorded in stockholders’ equity as a reduction of the proceeds generated from the related offering when it is completed. Deferred offering costs were $62 and $116 as of December 31, 2017 and 2016, respectively.

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation. Depreciation expense is recognized using the straight-line method over the useful life of the asset. Computer equipment is depreciated over three years. Manufacturing and laboratory equipment is depreciated over five years. Furniture and fixtures are depreciated over five years. Leasehold improvements are depreciated over the shorter of the lease term or their useful life. Expenditures for repairs and maintenance of assets are charged to expense as incurred. Upon retirement or sale, the cost and related accumulated depreciation of assets disposed of are removed from the accounts and any resulting gain or loss is included in loss from operations.

Impairment of Long Lived Assets

Long-lived assets consist of property and equipment. Long-lived assets to be held and used are tested for recoverability whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. Factors that the Company considers in deciding when to perform an impairment review include significant underperformance of the business in relation to expectations, significant negative industry or economic trends and significant changes or planned changes in the use of the assets. If an impairment review is performed to evaluate a long lived asset for recoverability, the Company compares forecasts of undiscounted cash flows expected to result from the use and eventual disposition of the long lived asset to its carrying value. An impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of an asset are less than its carrying amount. The impairment loss would be based on the excess of the carrying value of the impaired asset over its fair value, determined based on discounted cash flows.

Intangible Assets

Intangible assets include both finite-lived and indefinite-lived assets. Finite-lived intangible assets are amortized over their estimated useful life based on the pattern over which the intangible assets are consumed or otherwise used up. If that pattern cannot be reliably determined, the straight-line method of amortization is used. Finite-lived intangible assets consist of a research technology platform the Company acquired through the acquisition of Confluence. Indefinite-lived intangible assets consist of an in-process research and development (“IPR&D”) drug candidate acquired through the acquisition of Confluence. IPR&D assets are considered indefinite-lived until the completion or abandonment of the associated research and development efforts. The cost of IPR&D assets is either amortized over their estimated useful life beginning when the underlying drug candidate is approved and launched commercially, or expensed immediately if development of the drug candidate is abandoned.

Finite-lived intangible assets are tested for impairment when events or changes in circumstances indicate that the carrying value of the asset may not be recoverable. Indefinite-lived intangible assets are tested for impairment at least annually, which the Company performs during the fourth quarter, or when indicators of an impairment are present. The Company recognizes impairment losses when and to the extent that the estimated fair value of an indefinite-lived intangible asset is less than its carrying value.

Goodwill

Goodwill is not amortized, but rather is subject to testing for impairment at least annually, which the Company performs during the fourth quarter, or when indicators of an impairment are present. The Company considers each of its operating segments, dermatology therapeutics and contract research, to be a reporting unit since this is the lowest level for which discrete financial information is available. The Company has attributed the full amount of the goodwill acquired with Confluence, or $18,504, to the dermatology therapeutics segment. The annual impairment test performed by the Company is a qualitative assessment based upon current facts and circumstances related to operations of the dermatology therapeutics segment. If the qualitative assessment indicates an impairment may be present, the Company would perform the required quantitative analysis and an impairment charge would be recognized to the extent that the estimated fair value of the reporting unit is less than its carrying amount. However, any loss recognized would not exceed the total amount of goodwill allocated to that reporting unit.

Contingent Consideration

The Company initially recorded the contingent consideration related to future potential payments based upon the achievement of certain development, regulatory and commercial milestones, resulting from the acquisition of Confluence, at its estimated fair value on the date of acquisition. Changes in fair value reflect new information about the likelihood of the payment of the contingent consideration and the passage of time. Future changes in the fair value of the contingent consideration, if any, will be recorded as income or expense in the Company’s consolidated statement of operations.

Segment Data

The Company operates in two segments, dermatology therapeutics and contract research, for the purposes of assessing performance and making operating decisions. The Company’s dermatology therapeutics segment, which has not generated any revenue to date, is focused on identifying, developing and commercializing innovative and differentiated therapies to address significant unmet needs in medical and aesthetic dermatology and immunology. The Company’s contract research segment is focused on providing laboratory services under contract research arrangements to pharmaceutical and biotech companies looking to supplement their research and development efforts with difficult-to-execute specialty skills and programs.

Recently Issued and Adopted Accounting Pronouncements

In January 2017, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2017-01, Business Combinations-Clarifying the Definition of a Business (Topic 805). The amendments in this ASU provide a screen to determine when a set of acquired assets and/or activities is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired, or disposed of, is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. The amendments in this ASU will reduce the number of transactions that meet the definition of a business. ASU 2017-01 is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those years, and early adoption is permitted. The Company is assessing the potential impact of ASU 2017-01 on its consolidated financial statements.

In January 2017, the FASB issued ASU 2017-04, Intangibles-Goodwill and Other-Simplifying the Test for Goodwill Impairment (Topic 350). Under the amendments in this ASU, an entity should perform its annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. The amendments in this ASU eliminate Step 2 from the goodwill impairment test. ASU 2017-04 is effective for fiscal years beginning after December 15, 2019, and early adoption is permitted. The Company adopted the provisions of this standard early, the impact of which on its consolidated financial statements was not significant.

In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326). This ASU introduces a new model for recognizing credit losses on financial instruments based upon estimated expected credit losses. ASU 2016-13 will apply to loans, accounts receivable, financial assets measured at amortized cost and at fair value through other comprehensive income, loan commitments and certain off-balance sheet credit exposures. ASU 2016-13 is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those years, and early adoption is permitted. The Company is assessing the potential impact of ASU 2016-13 on its consolidated financial statements.

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606). Under this ASU, entities should recognize revenue in an amount that reflects the consideration to which they expect to be entitled to in exchange for goods and services provided. ASU 2014-09 is effective for annual reporting periods beginning after December 15, 2017. The Company is assessing the potential impact of ASU 2014-09 on its consolidated financial statements.

In March 2016, the FASB issued ASU 2016-09, Improvements to Employee Share-Based Payment Accounting. This ASU requires all tax effects of share-based payment settlements to be recorded through the income statement. Currently, tax benefits in excess of compensation cost, or “windfalls”, are recorded in equity, and tax deficiencies, or “shortfalls”, are recorded to equity to the extent of previous windfalls, and then to the income statement. In addition, under the new guidance, companies will be permitted to make a policy election to recognize the impact of forfeitures either when they occur, or on an estimated basis. Finally, this update simplifies withholding requirements to allow companies to withhold up to an employee’s maximum tax rate without resulting in liability classification for the award. ASU 2016-09 was effective for annual reporting periods beginning after December 15, 2016. The Company adopted the provisions of this standard early, the impact of which on its consolidated financial statements was not significant.

In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842). The new standard establishes a right-of-use (“ROU”) model that requires a lessee to record a ROU asset and a lease liability on the balance sheet for all leases with terms longer than 12 months. Leases will be classified as either finance or operating, with classification affecting the pattern of expense recognition in the income statement. ASU 2016-02 is effective for annual periods beginning after December 15, 2018, including interim periods within those annual periods, with early adoption permitted. A modified retrospective transition approach is required for lessees for capital and operating leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. The Company is currently evaluating the potential impact of the adoption of this standard.

In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities. The amendments in this update revise the accounting related to the classification and measurement of investments in equity securities and the presentation of certain fair value changes for financial liabilities measured at fair value. The amendments are effective for annual reporting periods beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the potential impact of the adoption of this standard.

In November 2015, the FASB issued ASU 2015- 17, Balance Sheet Classification of Deferred Taxes. The amendments in this update simplify the presentation of deferred income taxes to require that deferred tax liabilities and assets are classified as noncurrent in a statement of financial position. The amendments are effective for annual reporting periods beginning after December 15, 2016 and interim reporting periods within those annual periods. Early application is permitted. The Company adopted the provisions of this standard early, the impact of which on its consolidated financial statements was not significant.

Confluence Acquisition

12 Months Ended

Dec. 31, 2017

Confluence Acquisition

Confluence Acquisition

3. Confluence Acquisition

In August 2017, the Company entered into an Agreement and Plan of Merger with Confluence, Aclaris Life Sciences, Inc., a Delaware corporation and wholly-owned subsidiary of the Company (the “Merger Sub”), and Fortis Advisors LLC, as representative of the holders of Confluence equity (the “Agreement and Plan of Merger”). Pursuant to the terms of the Agreement and Plan of Merger, the Merger Sub merged with and into Confluence, with Confluence surviving as a wholly-owned subsidiary of the Company, resulting in the Company’s acquisition of 100% of the outstanding shares of Confluence. Pursuant to the terms of the Agreement and Plan of Merger, the Company gave aggregate consideration with a fair value of $24,322 to the equity holders of Confluence, subject to a post-closing working capital adjustment. Confluence was a privately held biotechnology company focused on the discovery and development of kinase inhibitors to treat inflammatory and immunological disorders and cancer. Confluence also provided laboratory services under contract research arrangements to pharmaceutical and biotechnology companies looking to supplement their research and development efforts with difficult-to-execute specialty skills and programs. The acquisition of Confluence has added small molecule drug discovery and preclinical development capabilities, which has allowed the Company to bring early-stage research and development activities in-house that were previously outsourced to third parties.

The Company also agreed to pay the Confluence equity holders contingent consideration of up to $80,000, based upon the achievement of certain development, regulatory and commercial milestones set forth in the Agreement and Plan of Merger. Of the contingent consideration, $2,500 may be paid in shares of the Company’s common stock upon the achievement of a specified development milestone. In addition, the Company has agreed to pay the Confluence equity holders specified future royalty payments calculated as a low single-digit percentage of annual net sales, subject to specified reductions, limitations and other adjustments, until the date that all of the patent rights for that product have expired, as determined on a country-by-country and product-by-product basis or, in specified circumstances, ten years from the first commercial sale of such product. In addition, if the Company sells, licenses or transfers any of the intellectual property acquired from Confluence pursuant to the Agreement and Plan of Merger to a third party, the Company will be obligated to pay the Confluence equity holders a portion of any incremental consideration (in excess of the development and milestone payments described above) that the Company receives from such sales, licenses or transfers in specified circumstances.

The following table summarizes the fair value of total consideration given to the Confluence equity holders pursuant to the Agreement and Plan of Merger:


Cash consideration paid	$	10,269
Aclaris common stock issued		9,675
Contingent consideration		4,378
Total fair value of consideration to Confluence equity holders	$	24,322

The Company funded the acquisition and transaction expenses with cash on hand.

The Company accounted for this transaction as a business combination using the acquisition method of accounting. Under the acquisition method of accounting, the assets acquired and liabilities assumed in this transaction were recorded at their respective fair values on the date of acquisition using assumptions that are subject to change. The Company expects to finalize its allocation of the purchase price upon the finalization of valuations for the identified intangible assets, final resolution of the post-closing working capital adjustment and certain tax accounts that are based on the best estimates of management. The completion and filing of federal and state tax returns for the acquired entity may result in adjustments to the carrying value of assets and liabilities.

The following table summarizes the preliminary fair value of assets acquired and liabilities assumed in the acquisition of Confluence as of the acquisition date:


Cash and cash equivalents	$	622
Accounts receivable, net		574
Other current assets		89
Property and equipment		268
Other intangible assets		751
IPR&D		6,629
Goodwill		18,504
Total assets acquired		27,437

Accounts payable and accrued expenses		656
Deferred tax liability		2,386
Other liabilities		73
Total liabilities assumed		3,115

Total net assets acquired	$	24,322

The estimated fair value of the IPR&D, and other identified intangibles, acquired was determined using a replacement cost method, which estimates the cost that would be required to rebuild the intangible assets identified in the acquisition of Confluence. The acquisition of Confluence resulted in the recognition of goodwill in the amount of $18,504 which represents the value of new products and technologies to be developed in the future as well as the value of the employee workforce acquired.

The following supplemental unaudited pro forma information presents the Company’s financial results, for the periods presented, as if the acquisition of Confluence had occurred on January 1, 2015. This supplemental unaudited pro forma financial information has been prepared for comparative purposes only, and is not necessarily indicative of what actual results would have been had the acquisition of Confluence occurred on January 1, 2015, nor is this information indicative of future results.


	Year Ended
	December 31,
	2017		2016		2015

Revenue	$	4,365	$	3,693	$	2,630
Gross profit		1,347		1,652		1,302
Total operating expenses		73,810		51,277		24,151
Net loss		(70,391)		(49,148)		(22,803)

The supplemental unaudited pro forma financial results for the year ended December 31, 2017 includes an adjustment to exclude $1,351 of acquisition-related expenses, as well as $888 to exclude revenue billed to the Company by Confluence. The supplemental unaudited pro forma financial results for the year ended December 31, 2017 also includes an adjustment for amortization expense related to the other intangible assets acquired.

There were no acquisition-related expenses incurred, or revenue billed to the Company by Confluence, for the years ended December 31, 2016 and 2015, and accordingly, no adjustments are necessary for these items in the supplemental pro forma financial results for those years. The supplemental unaudited pro forma financial results for the years ended December 31, 2016 and 2015 includes an adjustment for amortization expense related to the other intangible assets acquired.

Fair Value of Financial Assets and Liabilities

12 Months Ended

Dec. 31, 2017

Fair Value of Financial Assets and Liabilities

Fair Value of Financial Assets and Liabilities

4. Fair Value of Financial Assets and Liabilities

The following tables present information about the fair value measurements of the Company’s financial assets and liabilities which are measured at fair value on a recurring basis, and indicate the level of the fair value hierarchy utilized to determine such fair values:


	December 31, 2017
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	19,339	$	—	$	—	$	19,339
Marketable securities		—		188,652		—		188,652
Total Assets	$	19,339	$	188,652	$	—	$	207,991


Liabilities:
Acquisition-related contingent consideration	$	—	$	—	$	4,378	$	4,378
Total liabilities	$	—	$	—	$	4,378	$	4,378


	December 31, 2016
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	11,522	$	12,691	$	—	$	24,213
Marketable securities		—		143,963		—		143,963
Total	$	11,522	$	156,654	$	—	$	168,176

As of December 31, 2017 and 2016, the Company’s cash equivalents consisted of investments with maturities of less than three months and included a money market fund which was valued based upon Level 1 inputs, and commercial paper and corporate debt securities which were valued based upon Level 2 inputs. In determining the fair value of its Level 2 investments the Company relied on quoted prices for identical securities in markets that are not active. These quoted prices were obtained by the Company with the assistance of a third‑party pricing service based on available trade, bid and other observable market data for identical securities. Quarterly, the Company compares the quoted prices obtained from the third‑party pricing service to other available independent pricing information to validate the reasonableness of the quoted prices provided. The Company evaluates whether adjustments to third-party pricing is necessary and, historically, the Company has not made adjustments to quoted prices obtained from the third-party pricing service. During the years ended December 31, 2017 and 2016, there were no transfers between Level 1, Level 2 and Level 3.

As of December 31, 2017 and 2016, the fair value of the Company’s available-for-sale marketable securities by type of security was as follows:


	December 31, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gain		Loss		Value

Marketable securities:
Corporate debt securities	$	37,401	$	—	$	(68)	$	37,333
Commercial paper		85,202		—		—		85,202
Asset-backed securities		16,708		—		(13)		16,695
U.S. government agency debt securities		49,511		—		(89)		49,422
Total marketable securities	$	188,822	$	—	$	(170)	$	188,652


	December 31, 2016
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gain		Loss		Value
Marketable securities:
Corporate debt securities	$	51,352	$	—	$	(59)	$	51,293
Commercial paper		20,463		—		—		20,463
Asset-backed securities		28,692		6		(1)		28,697
U.S. government agency debt securities		43,505		8		(3)		43,510
Total marketable securities	$	144,012	$	14	$	(63)	$	143,963

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment, Net

12 Months Ended

Dec. 31, 2017

Property and Equipment, Net

Property and Equipment, Net

5. Property and Equipment, Net

Property and equipment, net consisted of the following:


	December 31,
	2017		2016
Computer equipment	$	650	$	310
Manufacturing equipment		511		149
Lab equipment		721		—
Furniture and fixtures		327		115
Leasehold improvements		430		33
Property and equipment, gross		2,639		607
Accumulated depreciation		(480)		(126)
Property and equipment, net	$	2,159	$	481

Depreciation expense was $370, $120 and $90 for the years ended December 31, 2017, 2016 and 2015, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2017

Accrued Expenses

Accrued Expenses

6. Accrued Expenses

Accrued expenses consisted of the following:


	December 31,
	2017		2016
Employee compensation expenses	$	3,010	$	1,732
Research and development expenses		627		1,166
Payable to NST		590		—
Vixen contract payable		100		100
Capital leases, current portion		142		—
Other		471		380
Total accrued expenses	$	4,940	$	3,378

Stockholders' Equity	12 Months Ended
Stockholders' Equity	Dec. 31, 2017
Stockholders' Equity
Stockholders' Equity	7. Stockholders’ Equity Preferred Stock As of December 31, 2017 and 2016, the Company’s amended and restated certificate of incorporation authorized the Company to issue 10,000,000 shares of undesignated preferred stock. There were no shares of preferred stock outstanding as of December 31, 2017 and 2016. The Company previously issued Series A, Series B and Series C convertible preferred stock (collectively, the “Preferred Stock”). Through August 28, 2015, the Company had issued an aggregate of 11,677,076 shares of Preferred Stock in the Series A, Series B and Series C offerings. Upon the closing of the Company’s IPO in October 2015, all of the then outstanding Preferred Stock was converted into an aggregate total of 11,677,076 shares of common stock. Also in connection with the IPO, the Company amended and restated its certificate of incorporation and authorized 10,000,000 shares of undesignated preferred stock. Common Stock As of December 31, 2017 and 2016, the Company’s certificate of incorporation, as amended and restated, authorized the Company to issue 100,000,000 shares of $0.00001 par value common stock. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to any preferential dividend rights of any series of preferred stock that may be outstanding. No dividends have been declared through December 31, 2017. Restricted Common Stock In July 2012, the Company issued 2,730,427 shares of restricted common stock with time-based vesting to its founders in connection with the formation of the Company. Unvested shares of restricted common stock could not be sold or transferred by the holders of those shares. Of the shares issued in July 2012, 1,918,834 shares were subject to vesting pursuant to restricted stock agreements with 25% vesting in July 2013 and the remaining 75% vesting in equal monthly installments over a three-year period thereafter. Upon the Company’s IPO in October 2015, all remaining unvested shares of restricted common stock vested immediately. The estimated grant‑date fair value of the restricted common stock issued was $0.00001 per share, equal to the par value of each share issued. The aggregate fair value of restricted common stock that vested during the years ended December 31, 2017, 2016 and 2015 was $0, $0 and $6,423, respectively. As of December 31, 2017 and 2016, no shares were subject to repurchase. Initial Public Offering In October 2015, the Company’s registration statement on Form S-1 relating to its initial public offering of its common stock (the “IPO”) was declared effective by the Securities and Exchange Commission (“SEC”). The Company’s common stock began trading on the Nasdaq Global Select Market on October 7, 2015. The IPO closed on October 13, 2015, and 5,000,000 shares of common stock were sold at a price to the public of $11.00 per share, for aggregate gross proceeds of $55,000. In addition, upon the closing of the IPO, all of the Company’s outstanding convertible preferred stock was converted into an aggregate total of 11,677,076 shares of common stock. The conversion of the convertible preferred stock was a non-cash transaction which has been excluded from the Consolidated Statements of Cash Flows. On October 12, 2015, the underwriters of the IPO exercised in full their option to purchase additional shares, and on October 13, 2015, the Company sold 750,000 additional shares of common stock at a price to the public of $11.00 per share, for aggregate gross proceeds of $8,250. The Company paid underwriting discounts and commissions of $4,428 to the underwriters in connection with the IPO, including the underwriters’ exercise of their option to purchase additional shares. In addition, the Company incurred expenses of $2,272 in connection with the IPO. The net offering proceeds received by the Company, after deducting underwriting discounts, commissions and offering expenses, were $56,550. Private Placement In June 2016, pursuant to a securities purchase agreement with certain accredited investors dated May 27, 2016, the Company closed a private placement in which it sold an aggregate of 1,081,082 shares of common stock at a price of $18.50 per share, for gross proceeds of $20,000. The Company incurred placement agent fees of $1,300 and expenses of $153 in connection with the private placement. The net offering proceeds received by the Company, after deducting placement agent fees and transaction expenses, were $18,547. November 2016 Public Offering In November 2016, the Company’s registration statement on Form S-3 was declared effective by the SEC. On November 23, 2016, the Company closed a follow-on public offering in which 4,000,000 shares of common stock were sold to the public at a price of $22.75 per share, for gross proceeds of $91,000. On November 17, 2016, the underwriters exercised in full their option to purchase 600,000 additional shares of common stock at a price to the public of $22.75 per share, for gross proceeds of $13,650. The Company paid underwriting discounts and commissions of $6,279 to the underwriters in connection with the offering, including the underwriters’ exercise of their option to purchase additional shares. In addition, the Company incurred expenses of $188 in connection with the offering. The net offering proceeds received by the Company, after deducting underwriting discounts, commissions and offering expenses, were $98,158. At-The-Market Equity Offering In November 2016, the Company entered into an at-the-market sales agreement with Cowen and Company, LLC to sell the Company’s securities under the Company’s registration statement on Form S-3. During the year ended December 31, 2017, the Company issued 635,000 shares of common stock under the at-the-market sales agreement. As of December 31, 2017, the Company had issued and sold an aggregate of 635,000 shares of common stock under the at-the-market sales agreement at a weighted average price per share of $31.50, for aggregate gross proceeds of $20,003. The Company incurred expenses of $691 in connection with the shares issued under the at-the-market sales agreement. August 2017 Public Offering In August 2017, the Company entered into an underwriting agreement pursuant to which the Company issued and sold 3,747,602 shares of common stock under the Company’s registration statement on Form S-3, including the underwriters’ partial exercise of their option to purchase additional shares. The shares of common stock were sold to the public at a price of $23.02 per share, for gross proceeds of $86,270. The Company paid underwriting discounts and commissions of $5,176 to the underwriters in connection with the offering. In addition, the Company incurred expenses of $176 in connection with the offering. The net offering proceeds received by the Company, after deducting underwriting discounts and commissions and offering expenses, were $80,918.

Stock-Based Awards

12 Months Ended

Dec. 31, 2017

Stock-Based Awards

Stock-Based Awards

8. Stock‑Based Awards

2017 Inducement Plan

In July 2017, the Company’s board of directors adopted the 2017 Inducement Plan (the “2017 Inducement Plan”). The 2017 Inducement Plan is a non-shareholder approved stock plan adopted pursuant to the “inducement exception” provided under Nasdaq listing rules. The only employees eligible to receive grants of awards under the 2017 Inducement Plan are individuals who satisfy the standards for inducement grants under Nasdaq rules, generally including individuals who were not previously an employee or director of the Company. Under the terms of the 2017 Inducement Plan the Company may grant up to 1,000,000 shares of common stock pursuant to nonqualified stock options, stock appreciation rights, restricted stock awards, RSUs, and other stock awards. The shares of common stock underlying any awards that expire, or are otherwise terminated, settled in cash or repurchased by the Company under the 2017 Inducement Plan will be added back to the shares of common stock available for issuance under the 2017 Inducement Plan. As of December 31, 2017, 489,884 shares of common stock were available for grant under the 2017 Inducement Plan.

2015 Equity Incentive Plan

In September 2015, the Company’s board of directors adopted the 2015 Equity Incentive Plan (the “2015 Plan”), and the Company’s stockholders approved the 2015 Plan. The 2015 Plan became effective in connection with the Company’s IPO. Beginning at the time the 2015 Plan became effective, no further grants may be made under the Company’s 2012 Equity Compensation Plan, as amended and restated (the “2012 Plan”). The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, RSU awards, performance stock awards, cash-based awards and other stock-based awards. The number of shares initially reserved for issuance under the 2015 Plan was 1,643,872 shares of common stock. The number of shares of common stock that may be issued under the 2015 Plan will automatically increase on January 1 of each year, beginning on January 1, 2016 and ending on January 1, 2025, in an amount equal to the lesser of (i) 4.0% of the shares of the Company’s common stock outstanding on December 31 of the preceding calendar year or (ii) an amount determined by the Company’s board of directors. The shares of common stock underlying any awards that expire, are otherwise terminated, settled in cash or repurchased by the Company under the 2015 Plan and the 2012 Plan will be added back to the shares of common stock available for issuance under the 2015 Plan. As of December 31, 2017, 1,404,498 shares remained available for grant under the 2015 Plan. As of January 1, 2018, the number of shares of common stock that may be issued under the 2015 Plan was automatically increased by 1,234,260 shares.

2012 Equity Compensation Plan

Upon the 2015 Plan becoming effective, no further grants can be made under the 2012 Plan. The Company granted a total of 1,140,524 stock options under the 2012 Plan, of which 984,720 and 1,049,667 were outstanding as of December 31, 2017, and 2016, respectively. Stock options granted under the 2012 Plan vest over four years and expire after ten years. As required, the exercise price for the stock options granted under the 2012 Plan was not less than the fair value of common shares as determined by the Company as of the date of grant.

Stock Option Valuation

The weighted average assumptions the Company used to estimate the fair value of stock options granted during the years ended December 31, 2017, 2016 and 2015 were as follows:


	Year Ended
	December 31,
	2017		2016		2015

Risk-free interest rate	1.93	%	2.06	%	1.78	%
Expected term (in years)	6.2		6.5		6.3
Expected volatility	94.19	%	94.86	%	93.90	%
Expected dividend yield	0	%	0	%	0	%

The Company recognizes compensation expense for awards over their vesting period. Compensation expense for awards includes the impact of forfeiture in the period when they occur.

Stock Options

The following table summarizes stock option activity for the years ended December 31, 2017, 2016 and 2015:


					Weighted
			Weighted		Average
			Average		Remaining	Aggregate
	Number		Exercise		Contractual	Intrinsic
	of Shares		Price		Term	Value
					(in years)
Outstanding as of December 31, 2014	500,262		$	1.22	9.77	$	305
Granted	1,238,262			18.08
Exercised	—			—
Forfeited and canceled	—			—
Outstanding as of December 31, 2015	1,738,524			13.23	9.51		24,722
Granted	1,083,919			27.12
Exercised	(51,980)			—
Forfeited and canceled	(68,113)			—
Outstanding as of December 31, 2016	2,702,350		$	18.94	9.05	$	24,434
Granted	790,100			26.21
Exercised	(36,738)			6.40
Forfeited and cancelled	(126,955)			22.05
Outstanding as of December 31, 2017	3,328,757		$	20.69	8.28	$	19,812
Options vested and expected to vest as of December 31, 2017	3,328,757		$	20.69	8.28	$	19,812
Options exercisable as of December 31, 2017	1,239,736	(1)	$	15.56	7.43	$	13,414

(1)

All options granted under the 2012 Plan are exercisable immediately, subject to a repurchase right in the Company’s favor that lapses as the option vests. This amount reflects the number of shares under options that were vested, as opposed to exercisable, as of December 31, 2017.

The weighted average grant date fair value of stock options granted during the years ended December 31, 2017, 2016 and 2015 was $20.28, $21.16 and $13.84 per share, respectively.

The intrinsic value of a stock option is calculated as the difference between the exercise price of the stock option and the fair value of the underlying common stock, and cannot be less than zero.

Restricted Stock Units

The following table summarizes RSU activity for the years ended December 31, 2017, 2016 and 2015.


		Weighted
		Average
		Grant Date
	Number	Fair Value
	of Shares	Per Share
Outstanding as of December 31, 2014	—
Granted	53,800	$	28.68
Vested	—
Forfeited and cancelled	—
Outstanding as of December 31, 2015	53,800		28.68
Granted	180,764		27.16
Vested	(12,950)		28.68
Forfeited and cancelled	(2,000)		28.68
Outstanding as of December 31, 2016	219,614	$	27.43
Granted	117,883		26.27
Vested	(40,705)		26.89
Forfeited and cancelled	(13,239)		27.53
Outstanding as of December 31, 2017	283,553	$	27.02

Stock‑Based Compensation

The following table summarizes stock-based compensation expense recorded by the Company for the years ended December 31, 2017, 2016 and 2015:


	Year Ended
	December 31,
	2017		2016		2015

Cost of revenue	$	211	$	—	$	—
Research and development		5,471		2,291		257
General and administrative		8,748		3,813		634
Total stock-based compensation expense	$	14,430	$	6,104	$	891

As of December 31, 2017, the Company had unrecognized stock‑based compensation expense for stock options and RSUs of $36,150 and $5,849, respectively, which is expected to be recognized over weighted average periods of 2.93 years and 2.93 years, respectively.

Net Loss per Share

12 Months Ended

Dec. 31, 2017

Net Loss per Share

Net Loss per Share

9. Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders was calculated as follows:


	Year Ended
	December 31,
	2017		2016		2015
Numerator:
Net loss	$	(68,523)	$	(48,079)	$	(20,563)
Accretion of redeemable convertible preferred stock		—		—		(2,566)
Net loss attributable to common stockholders	$	(68,523)	$	(48,079)	$	(23,129)
Denominator:
Weighted average shares of common stock outstanding		28,102,386		21,415,733		6,637,678
Less: Weighted average shares of unvested restricted common stock outstanding		—		—		(530,636)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders, basic and diluted		28,102,386		21,415,733		6,107,042
Net loss per share attributable to common stockholders, basic and diluted	$	(2.44)	$	(2.25)	$	(3.79)

To calculate net loss attributable to common stockholders the Company reduced the net loss for the accretion of issuance costs and cumulative dividends accrued but not paid through August 28, 2015, and the remaining cumulative dividends accrued but not paid through October 13, 2015, the date on which all convertible preferred stock converted to common stock.

The Company’s potential dilutive securities, which included stock options, RSUs, preferred stock, and shares of restricted common stock that were issued but not yet vested, have been excluded from the computation of diluted net loss per share since the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders is the same. The following table presents potential common shares excluded from the calculation of diluted net loss per share attributable to common stockholders for the years ended December 31, 2017, 2016 and 2015. All share amounts presented in the table below represent the total number outstanding as of December 31.


	Year Ended
	December 31,
	2017	2016	2015

Options to purchase common stock	3,328,757	2,702,350	1,738,524
Restricted stock unit awards	283,553	219,614	53,800
Total potential common shares	3,612,310	2,921,964	1,792,324

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Commitments and Contingencies

10. Commitments and Contingencies

Agreements for Office Space

In November 2017, the Company entered into a sublease agreement with a third party. The Company subleases 33,019 square feet of office space under the terms of the agreement for its headquarters in Wayne, Pennsylvania. Subject to the consent of Chesterbrook Partners, LP (“Landlord”) as set forth in the lease by and between them and Auxilium Pharmaceuticals, LLC (“Sublandlord”), the sublease has a term that runs through October 2023. If for any reason the lease between the Landlord and Sublandlord is terminated or expires prior to October 2023, the Company’s sublease will automatically terminate.

In November 2016, the Company entered into a lease agreement with a third party for additional office space in the Malvern, Pennsylvania with a term beginning in February 2017, and ending in November 2019. The Company also occupies office and laboratory space in St. Louis, Missouri under the terms of an agreement which it entered into in January 2018 and which expires in December 2018.

Rent expense was $946, $254 and $119 for the years ended December 31, 2017, 2016 and 2015, respectively. The Company recognizes rent expense on a straight-line basis over the term of the agreement and has accrued for rent expense incurred but not yet paid.

As of December 31, 2017, future minimum lease payments under the sublease were as follows:


Year Ending December 31,
2018	$	664
2019		627
2020		589
2021		605
2022		622
Thereafter		532
Total	$	3,639

Capital Leases for Laboratory Equipment

The Company leases laboratory equipment which is used in its laboratory space in St. Louis, Missouri under two capital lease financing arrangements which the Company entered into in August 2017 and October 2017. The capital leases have terms which end in October 2020 and December 2020.

Stock Purchase Agreement with Vixen Pharmaceuticals, Inc

Pursuant to the stock purchase agreement with Vixen the Company is obligated to make annual payments of $100 on March 24th of each year, through March 2022, with such amounts being creditable against specified future payments.

Indemnification Agreements

In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its consolidated financial statements as of December 31, 2017 or 2016.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Taxes

Income Taxes

11. Income Taxes

The Tax Cuts and Jobs Act (the "TCJA") was enacted on December 22, 2017 and became effective January 1, 2018. The Tax Act made significant changes to U.S. tax law, including lowering U.S. corporate income tax rates, implementing a territorial tax system, imposing a one-time transition tax on deemed repatriated earnings of foreign subsidiaries and modifying the taxation of other income and expense items.

The TCJA reduces the U.S. corporate income tax rate from 35% to 21%, effective January 1, 2018. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to reverse. As a result of the reduction in the U.S. corporate income tax rate from 35% to 21% under the TCJA, the Company revalued its deferred tax liabilities, net as of December 31, 2017. The impact of revaluation of the deferred tax liabilities, net was $18,507 of income tax expense, which was more than offset by a reduction in the valuation allowance of $20,344 resulting in a net impact of a $1,837 tax benefit. The net tax benefit recorded was primarily the result of tax law changes which impacted the deferred tax liability the Company recorded for IPR&D related to the acquisition of Confluence. Under GAAP, IPR&D is an indefinite lived intangible that is capitalized on the balance sheet, but which does not have a cost basis under U.S. tax law.

The TCJA provided for a one-time transition tax on the deemed repatriation of post-1986 undistributed foreign subsidiary earnings and profits. The Company did not have consolidated accumulated earnings and profits attributable to its foreign subsidiary; accordingly, the Company did not record any income tax expense related to the transition tax.

Due to the timing of the enactment of the TCJA and the substantial changes it brings, the Staff of the SEC issued SAB 118 which provides a measurement period to report the impact of the TCJA. During the measurement period, provisional amounts for the effects of the law are recorded to the extent a reasonable estimate can be made. To the extent that all information necessary is not available, prepared or analyzed, companies may recognize provisional estimated amounts for a period of up to one year following enactment of the TCJA.

During the years ended December 31, 2017, 2016 and 2015, the Company did not record an income tax benefit for net operating losses incurred in each year due to the uncertainty of realizing a benefit from those items.

Loss before income taxes is allocated as follows:


	Year Ended December 31,
	2017		2016		2015
U.S. operations	$	(63,665)	$	(40,597)	$	(11,823)
Foreign operations		(6,688)		(7,482)		(8,740)
Loss before income taxes	$	(70,353)	$	(48,079)	$	(20,563)

A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows:


	Year Ended December 31,
	2017		2016		2015
Federal statutory income tax rate	(34.0)	%	(34.0)	%	(34.0)	%
State taxes, net of federal benefit	(9.7)		(5.2)		(3.8)
Research and development tax credits	(1.1)		(2.0)		(1.5)
Permanent differences	0.4		1.8		—
Foreign rate differential	1.7		3.2		6.0
Change in deferred tax asset valuation allowance	17.4		36.2		33.3
Impact of U.S. tax reform	22.7		—		—
Effective income tax rate	(2.6)	%	—	%	—	%

Deferred tax liabilities, net as of December 31, 2017 and 2016 consisted of the following:


		December 31,
	2017		2016
Deferred tax assets:
Net operating loss carryforwards	$	26,566	$	16,836
Capitalized start up costs		9,940		8,840
Research and development tax credit carryforwards		2,296		1,480
Capitalized research and development expenses		3,595		594
Intangible asset		—		1,403
Stock‑based compensation expenses		6,220		2,629
Property and equipment		86		199
Other		280		2
Total deferred tax assets		48,983		31,983
Deferred tax liabilities:
Section 481(a) adjustment		(498)		(1,257)
Intangible asset		(1,843)		—
Other		(313)		—
Total deferred tax liabilities		(2,654)		(1,257)
Valuation allowance		(46,878)		(30,726)
Deferred tax liabilities, net	$	(549)	$	—

As of December 31, 2017, the Company had federal and state net operating loss carryforwards of $76,310 and $117,808, respectively, which begin to expire in 2032. As of December 31, 2017, the Company also had federal research and development tax credit carryforwards of $2,202 which begin to expire in 2032, and state research and development tax credit carryforwards of $118 which begin to expire in 2022. The Company also has $1,292 of loss carry forwards in the United Kingdom which can be carried forward indefinitely. Utilization of the net operating loss carryforwards and research and development tax credit carryforwards in the United States may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986 due to ownership changes that may have occurred previously or that could occur in the future. These ownership changes may limit the amount of carryforwards that can be utilized annually to offset future taxable income. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain stockholders or public groups in the stock of a corporation by more than 50% over a three-year period. The Company has completed an analysis under Section 382 for NOLs generated from July 13, 2012 through December 31, 2016. Although the Company has experienced Section 382 ownership changes since 2012, the Company has concluded that it should have sufficient ability to utilize NOLs accumulated during the periods tested. The Company has not yet determined if a Section 382 ownership change has occurred during the year ended December 31, 2017, or for Confluence prior to the acquisition. In addition, the Company may experience ownership changes in the future as a result of subsequent shifts in its stock ownership, some of which may be outside of the Company’s control.

The Company has evaluated the positive and negative evidence bearing upon its ability to realize the deferred tax assets. Management has considered the Company’s history of cumulative net losses incurred since inception and its lack of commercialization of any products or generation of any revenue from product sales since inception and has concluded that it is more likely than not that the Company will not realize the benefits of the deferred tax assets. Accordingly, a full valuation allowance has been established against the deferred tax assets as of December 31, 2017 and 2016. The Company evaluates positive and negative evidence of its’ ability to realize deferred tax assets at each reporting period.

Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017, 2016, and 2015 related primarily to the increases in net operating loss carryforwards, capitalized start-up costs, and research and development tax credit carryforwards and were as follows:


		Year Ended December 31,
		2017		2016		2015
Valuation allowance at beginning of year	$	(30,726)	$	(13,286)	$	(6,444)
Decreases recorded as benefit to income tax provision		—		—		—
Increases resulting from the acquisition of Confluence		(4,176)		—		—
Increases recorded to income tax provision		(11,976)		(17,440)		(6,842)
Valuation allowance as of end of year	$	(46,878)	$	(30,726)	$	(13,286)

During the year ended December 31, 2015, the Company recorded unrecognized tax benefits in the amount of $4,400 related to start-up costs that were previously deducted beginning in the initial return filing period ended December 31, 2012. During the year ended December 31, 2016, the Company filed a method of accounting change with the IRS related to the start-up costs, and reversed the related unrecognized tax position. During the year ended December 31, 2017, the Company recorded uncertain tax benefits related to tax positions from the acquired Confluence business. The following table summarizes the changes in the Company’s unrecognized tax benefits:


	Year ended December 31,
	2017		2016		2015
Unrecognized tax benefits at beginning of year	$	—	$	(4,400)	$	—
Increases related to prior year tax provisions		(43)		—		(2,624)
Decreases related to prior year tax provisions		—		4,400		—
Increases related to current year tax provisions		—		—		(1,776)
Unrecognized tax benefits as of end of year	$	(43)	$	—	$	(4,400)

The total amount of unrecognized tax benefits that, if recognized, would impact the Company’s effective tax rate were $36 and $0 as of December 31, 2017 and 2016, respectively. The Company accrues interest and penalties related to unrecognized tax benefits in income tax expense (benefit) in the consolidated statements of operations and comprehensive loss. During each of the years ended December 31, 2017, 2016 and 2015, the Company recognized expense (benefit) of $3, $0 and $0, respectively, related to interest and penalties.

The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s tax years are still open under statute from 2012 to the present. All open years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. The Company’s policy is to record interest and penalties related to income taxes as part of its income tax provision.

Related Party Transactions

12 Months Ended

Dec. 31, 2017

Related Party Transactions

Related Party Transactions

12. Related Party Transactions

In August 2013, the Company entered into a sublease agreement with NeXeption, Inc. ("NeXeption"), which was subsequently amended and restated in March 2014 and further amended in December 2014. In August 2015, pursuant to an Assignment and Assumption Agreement, NeXeption, Inc. assigned all interests, rights, duties and obligations under the sublease to NST Consulting, LLC, a wholly-owned subsidiary of NST, LLC. Following the Assignment and Assumption Agreement, the sublease was further amended in August 2015, February 2016, October 2016 and July 2017. On November 30, 2017, the Company entered into an agreement with NST Consulting, LLC to terminate the sublease effective March 31, 2018. The Company agreed to pay $590 to NST Consulting, LLC, which amount represents accelerated rent payments. The Company recorded a one-time charge of $506 in the year ended December 31, 2017 which is included in general and administrative expenses in the consolidated statement of operations. Total payments made under the sublease during the years ended December 31, 2017, 2016 and 2015, were $318, $253 and $127, respectively.

In February 2014, the Company entered into a services agreement with NST, LLC (the “NST Services Agreement”), pursuant to which NST, LLC provided certain pharmaceutical development, management and other administrative services to the Company. The NST Services Agreement was amended in January 2015 pursuant to which NST, LLC assigned all interests, rights, duties and obligations under the NST Services Agreement to NST Consulting, LLC. Under the NST Services Agreement, as amended, the Company also provided services to another company under common control with the Company and NST Consulting, LLC and was reimbursed by NST Consulting, LLC for those services. The Company may offset any payments owed by the Company to NST Consulting, LLC against payments that are owed by NST Consulting, LLC to the Company for the provision of personnel, including consultants, to the Company. In November 2017, the Company provided notice of termination of the NST Services Agreement to NST Consulting, LLC effective December 31, 2017.

Mr. Stephen Tullman, the chairman of the Company’s board of directors, was an executive officer of NeXeption and is also the manager of NST Consulting, LLC and NST, LLC, and three of the Company’s executive officers are and have been members of entities affiliated with NST, LLC.

During the years ended December 31, 2017, 2016 and 2015 amounts included in the consolidated statement of operations for the NST Services Agreement are summarized in the following table:


	Year Ended
	December 31,
	2017		2016		2015
Services provided by NST Consulting, LLC	$	225	$	323	$	455
Services provided to NST Consulting, LLC		(17)		(56)		(294)
General and administrative expense, net	$	208	$	267	$	161

Services provided by NST Consulting, LLC	$	—	$	246	$	52
Services provided to NST Consulting, LLC		—		(97)		(259)
Research and development expense, net	$	—	$	149	$	(207)

Services provided by NST Consulting, LLC	$	225	$	569	$	507
Services provided to NST Consulting, LLC		(17)		(153)		(553)
Total, net	$	208	$	416	$	(46)


Net payments made to (received from) NST Consulting, LLC	$	300	$	325	$	(46)

The Company had a net amount payable of $570 and $91 due to NST Consulting, LLC under the NST Services Agreement as of December 31, 2017, and December 31, 2016, respectively.

Agreements Related to Intellectual Property	12 Months Ended
Agreements Related to Intellectual Property	Dec. 31, 2017
Agreements Related to Intellectual Property
Agreements Related to Intellectual Property	13. Agreements Related to Intellectual Property Assignment Agreement and Finder’s Services Agreement In August 2012, the Company entered into an assignment agreement with the Estate of Mickey Miller (the “Miller Estate”) under which the Company acquired some of the intellectual property rights covering A-101. The assignment of intellectual property rights covers specified know-how, along with modifications of, improvements to and variations on A-101 that meet defined chemical properties. Under the agreement, the Company has the sole and exclusive right, but not the duty, to develop, obtain regulatory approval for and commercialize A-101 in various countries throughout the world. The Company is required to use commercially reasonable efforts to develop and commercialize at least one product for at least one indication in the United States. In connection with obtaining the assignment of the intellectual property from the Miller Estate, the Company also entered into a separate finder’s services agreement with KPT Consulting, LLC. Under the terms of the assignment agreement and the finder’s services agreement, the Company made one-time milestone payments of $400 in 2013 upon the dosing of the first human subject with ESKATA in the Company’s Phase 2 clinical trial. There are no remaining potential milestone payments under the assignment agreement. Under the finder’s services agreement, the Company made one-time milestone payments of $300 in the year ended December 31, 2016 upon the dosing of the first human subject with ESKATA in the Company’s Phase 3 clinical trial and $1,000 in the year ended December 31, 2017 upon the achievement of specified regulatory milestones. The Company is obligated to make additional payments of up to $4,500 upon the achievement of specified commercial milestones under the finder’s services agreement. The Company recorded both milestone payments made under the finder’s services agreement as general and administrative expense in the consolidated statement of operations. Under each of the assignment agreement and the finder’s services agreement, the Company is also obligated to pay royalties on sales of ESKATA or related products, at low single-digit percentages of net sales, subject to reduction in specified circumstances. The Company has not made any royalty payments to date under either agreement. Both agreements will terminate upon the expiration of the last pending, viable patent claim of the patents acquired under the assignment agreement, but no sooner than 15 years from the effective date of the agreements. License Agreement with Rigel Pharmaceuticals, Inc. In August 2015, the Company entered into an exclusive, worldwide license and collaboration agreement with Rigel Pharmaceuticals, Inc. (“Rigel”) for the development and commercialization of products containing specified JAK inhibitors developed by Rigel. Under this agreement, the Company intends to develop these JAK inhibitors for the treatment of alopecia areata and potentially for other dermatological conditions. During the year ended December 31, 2015, the Company made an upfront non-refundable payment of $8,000 to Rigel. In addition, the Company has agreed to make aggregate payments of up to $80,000 upon the achievement of specified pre‑commercialization milestones, such as clinical trials and regulatory approvals. Further, the Company has agreed to pay up to an additional $10,000 to Rigel upon the achievement of a second set of development milestones. With respect to any products the Company commercializes under the agreement, the Company will pay Rigel quarterly tiered royalties on its annual net sales of each product at a high single‑digit percentage of annual net sales, subject to specified reductions, until the date that all of the patent rights for that product have expired, as determined on a country‑by‑country and product‑by‑product basis or, in specified countries under specified circumstances, ten years from the first commercial sale of such product. The agreement terminates on the date of expiration of all royalty obligations unless earlier terminated by either party for a material breach. The Company may also terminate the agreement without cause at any time upon advance written notice to Rigel. Rigel, after consultation with the Company, will be responsible for maintaining and prosecuting the patent rights, and the Company will have final decision making authority regarding such patent rights for a product in the United States and the European Union. To the extent that the Company and Rigel jointly develop intellectual property, the parties will confer and decide which party will be responsible for filing, prosecuting and maintaining those patent rights. The agreement also establishes a joint steering committee composed of an equal number of representatives for each party which will monitor progress in the development of products. The Company accounted for the transaction as an asset acquisition as the licensing arrangement did not meet the definition of a business pursuant to the guidance prescribed in ASC Topic 805, Business Combinations. Accordingly, the Company recorded the $8,000 upfront payment as research and development expense in the year ended December 31, 2015. The Company will record as expense any contingent milestone payments or royalties in the period in which such liabilities are incurred. The Company concluded that licensing arrangement with Rigel did not meet the definition of a business because the transaction principally resulted in its acquisition of intellectual property. As part of the transaction, the Company did not acquire any employees or tangible assets, or any processes, protocols or operating systems. In addition, at the time of the acquisition, there were no activities being conducted related to the licensed patents. The Company will expense the acquired intellectual property asset as of the acquisition date on the basis that costs of intangible assets that are purchased from others for use in research and development activities and that have no alternative future uses are research and development costs at the time the costs are incurred. Stock Purchase Agreement with Vixen Pharmaceuticals, Inc. and License Agreement with Columbia University In March 2016, the Company entered into a stock purchase agreement (the “Vixen Agreement”) with Vixen, JAK1, LLC, JAK2, LLC and JAK3, LLC (all together with Vixen, the “Selling Stockholders”) and Shareholder Representative Services LLC, a Colorado limited liability company, solely in its capacity as the representative of the Selling Stockholders. Pursuant to the Vixen Agreement, the Company acquired all shares of Vixen’s capital stock from the Selling Stockholders (the “Vixen Acquisition”). Following the Vixen Acquisition, Vixen became a wholly-owned subsidiary of the Company. Pursuant to the Vixen Agreement, the Company paid $600 upfront and issued an aggregate of 159,420 shares of the Company’s common stock to the Selling Stockholders. The Company is obligated to make annual payments of $100 on March 24th of each year, through March 2022, with such amounts being creditable against specified future payments that may be paid under the Vixen Agreement. The Company is obligated to make aggregate payments of up to $18,000 to the Selling Stockholders upon the achievement of specified pre-commercialization milestones for three products in the United States, the European Union and Japan, and aggregate payments of up to $22,500 upon the achievement of specified commercial milestones. With respect to any commercialized products covered by the Vixen Agreement, the Company is obligated to pay low single-digit royalties on net sales, subject to specified reductions, limitations and other adjustments, until the date that all of the patent rights for that product have expired, as determined on a country-by-country and product-by-product basis or, in specified circumstances, ten years from the first commercial sale of such product. If the Company sublicenses any of Vixen’s patent rights and know-how acquired pursuant to the Vixen Agreement, the Company will be obligated to pay a portion of any consideration the Company receives from such sublicenses in specified circumstances. As a result of the transaction with Vixen, the Company became party to the Exclusive License Agreement, by and between Vixen and the Trustees of Columbia University in the City of New York (“Columbia”), dated as of December 31, 2015 (the “License Agreement”). Under the License Agreement, the Company is obligated to pay Columbia an annual license fee of $10, subject to specified adjustments for patent expenses incurred by Columbia and creditable against any royalties that may be paid under the License Agreement. The Company is also obligated to pay up to an aggregate of $11,600 upon the achievement of specified commercial milestones, including specified levels of net sales of products covered by Columbia patent rights and/or know-how, and royalties at a sub-single-digit percentage of annual net sales of products covered by Columbia patent rights and/or know-how, subject to specified adjustments. If the Company sublicenses any of Columbia’s patent rights and know-how acquired pursuant to the License Agreement, it will be obligated to pay Columbia a portion of any consideration received from such sublicenses in specified circumstances. The royalties, as determined on a country-by-country and product-by-product basis, are payable until the date that all of the patent rights for that product have expired, the expiration of any market exclusivity period granted by a regulatory body or, in specified circumstances, ten years from the first commercial sale of such product. The License Agreement terminates on the date of expiration of all royalty obligations thereunder unless earlier terminated by either party for a material breach, subject to a specified cure period. The Company may also terminate the License Agreement without cause at any time upon advance written notice to Columbia. The Company accounted for the transaction with Vixen as an asset acquisition as the arrangement did not meet the definition of a business pursuant to the guidance prescribed in ASC Topic 805, Business Combinations. The Company concluded the transaction with Vixen did not meet the definition of a business because the transaction principally resulted in the acquisition of the License Agreement. The Company did not acquire tangible assets, processes, protocols or operating systems. In addition, at the time of the transaction, there were no activities being conducted related to the licensed patents. The Company expensed the acquired intellectual property as of the acquisition date on the basis that the cost of intangible assets purchased from others for use in research and development activities, and that have no alternative future uses, are expensed at the time the costs are incurred. Accordingly, the Company recorded the $600 upfront payment, the fair value of the shares of common stock issued of $2,355, and the present value of the six non-contingent annual payments as research and development expense in the year ended December 31, 2016. Additionally, the Company will record as expense any contingent milestone payments or royalties in the period in which such liabilities are incurred.

401(k) Savings Plan	12 Months Ended
401(k) Savings Plan	Dec. 31, 2017
Defined Contribution Plan [Abstract]
401(k) Savings Plan	14. 401(k) Savings Plan The Company has a defined contribution savings plan under Section 401(k) of the Internal Revenue Code. This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. Company contributions to the plan may be made at the discretion of the Company’s board of directors. The Company has elected to match 100% of employee contributions to the 401(k) Plan up to 4% of the employee’s earnings, subject to certain limitations. Company contributions under the 401(k) Plan were $270, $176 and $99 for the years ended December 31, 2017, 2016 and 2015, respectively.

Segment Information

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Segment Information

15. Segment Information

The Company has two reportable segments, dermatology therapeutics and contract research. The dermatology therapeutics segment is focused on identifying, developing and commercializing innovative and differentiated therapies to address significant unmet needs in medical and aesthetic dermatology. The Company’s lead drug, ESKATA, is a proprietary formulation of high-concentration hydrogen peroxide topical solution that the Company is commercializing as a prescription treatment for raised SKs, a common non-malignant skin tumor, and which will be distributed by a wholesaler. The contract research segment earns revenue from the provision of laboratory services to clients through Confluence, the Company’s wholly-owned subsidiary. Laboratory service revenue is generally evidenced by contracts with clients which are on an agreed upon fixed-price, fee-for-service basis. The Company does not report balance sheet information by segment since it is not reviewed by the chief operating decision maker, and all of the Company’s tangible assets are held in the United States.


	Dermatology		Contract		Corporate		Total
Year Ended December 31, 2017	Therapeutics		Research		and Other		Company
Revenue	$	—	$	3,202	$	(1,519)	$	1,683
Cost of revenue		—		2,726		(1,519)		1,207
Research and development		39,790		—		—		39,790
General and administrative		13,991		673		18,445		33,109
Loss from operations	$	(53,781)	$	(197)	$	(18,445)	$	(72,423)


	Dermatology		Contract		Corporate		Total
Year Ended December 31, 2016	Therapeutics		Research		and Other		Company
Revenue	$	—	$	—	$	—	$	—
Cost of revenue		—		—		—		—
Research and development		33,476		—		—		33,476
General and administrative		3,450		—		11,641		15,091
Loss from operations	$	(36,926)	$	—	$	(11,641)	$	(48,567)


	Dermatology		Contract		Corporate		Total
Year Ended December 31, 2015	Therapeutics		Research		and Other		Company
Revenue	$	—	$	—	$	—	$	—
Cost of revenue		—		—		—		—
Research and development		15,339		—		—		15,339
General and administrative		1,195		—		4,133		5,328
Loss from operations	$	(16,534)	$	—	$	(4,133)	$	(20,667)

Foreign Subsidiary

The Company’s wholly-owned subsidiary, ATIL, was formed and operates in the United Kingdom. ATIL is utilized for research and development, regulatory and administrative functions and had $175 and $4,786 of net assets, composed principally of cash, as of December 31, 2017 and 2016, respectively.

Intersegment Revenue

Revenue for the contract research segment includes $1,519 for services performed on behalf of the dermatology therapeutics segment for the period between August 3, 2017, the acquisition date for Confluence, and December 31, 2017. All intersegment revenue has been eliminated in the Company’s consolidated statement of operations.

Quarterly Financial Information (unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information [Text Block]

16. Quarterly Financial Information (unaudited)

The following table summarizes the unaudited consolidated financial results of operations for the quarters indicated:


	2017 Quarter Ended
	March 31,		June 30,		September 30,		December 31,
Revenue	$	—	$	—	$	684	$	999
Gross profit		—		—		231		245
Operating expenses		12,930		15,295		18,987		25,687
Other income, net		371		457		564		678
Net loss		(12,559)		(14,838)		(18,192)		(22,934)
Net loss per share, basic and diluted	$	(0.48)	$	(0.56)	$	(0.63)	$	(0.74)

	2016 Quarter Ended
	March 31,		June 30,		September 30,		December 31,
Revenue	$	—	$	—	$	—	$	—
Gross profit		—		—		—		—
Operating expenses		13,139		12,989		10,812		11,627
Other income, net		100		118		118		152
Net loss		(13,039)		(12,871)		(10,694)		(11,475)
Net loss per share, basic and diluted	$	(0.65)	$	(0.62)	$	(0.50)	$	(0.49)

Net loss per share is computed independently for each quarter and, therefore, the sum of the quarterly per share amounts may not equal the year-to-date per share amount.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates

Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.

Revenue Recognition

The Company recognizes revenue when the earnings process is complete, which under SEC Staff Accounting

Research and Development Costs

Foreign Currency Translation

Stock-Based Compensation

The fair value of each RSU is measured using the closing price of the Company’s common stock on the date of grant.

Patent Costs

Income Taxes

Reverse Stock Split

Accretion of Convertible Preferred Stock

Comprehensive Loss

Net Loss per Share

Cash Equivalents

Marketable Securities

Assets Held for Sale

Other Assets

Fair Value Measurements

Level 1 — Quoted prices in active markets for identical assets or liabilities.

Concentration of Credit Risk and of Significant Suppliers

Concentration of Credit Risk and of Significant Customers and Suppliers

The Company’s top five customers represented 70% of total laboratory service revenues earned from August 3, 2017, the date of acquisition of Confluence, through December 31, 2017.

Deferred Offering costs

Deferred Offering Costs

Property and Equipment

Impairment of Long-Lived Assets

Impairment of Long Lived Assets

Intangible Assets

Goodwill

Contingent consideration

Contingent Consideration

Segment Data

Recently Issued and Adopted Accounting Pronouncements

Confluence Acquisition (Tables) - Confluence

12 Months Ended

Dec. 31, 2017

Business Acquisition [Line Items]

Schedule of Consideration Transferred


Cash consideration paid	$	10,269
Aclaris common stock issued		9,675
Contingent consideration		4,378
Total fair value of consideration to Confluence equity holders	$	24,322

Schedule of Assets Acquired and Liabilities Assumed


Cash and cash equivalents	$	622
Accounts receivable, net		574
Other current assets		89
Property and equipment		268
Other intangible assets		751
IPR&D		6,629
Goodwill		18,504
Total assets acquired		27,437

Accounts payable and accrued expenses		656
Deferred tax liability		2,386
Other liabilities		73
Total liabilities assumed		3,115

Total net assets acquired	$	24,322

Schedule of Pro Forma Information


	Year Ended
	December 31,
	2017		2016		2015

Revenue	$	4,365	$	3,693	$	2,630
Gross profit		1,347		1,652		1,302
Total operating expenses		73,810		51,277		24,151
Net loss		(70,391)		(49,148)		(22,803)

Fair Value of Financial Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value of Financial Assets and Liabilities

Schedule of assets and liabilities measured at fair value on a recurring basis


	December 31, 2017
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	19,339	$	—	$	—	$	19,339
Marketable securities		—		188,652		—		188,652
Total Assets	$	19,339	$	188,652	$	—	$	207,991


Liabilities:
Acquisition-related contingent consideration	$	—	$	—	$	4,378	$	4,378
Total liabilities	$	—	$	—	$	4,378	$	4,378


	December 31, 2016
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	11,522	$	12,691	$	—	$	24,213
Marketable securities		—		143,963		—		143,963
Total	$	11,522	$	156,654	$	—	$	168,176

Schedule of the fair value of available for sale marketable securities by type of security


	December 31, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gain		Loss		Value

Marketable securities:
Corporate debt securities	$	37,401	$	—	$	(68)	$	37,333
Commercial paper		85,202		—		—		85,202
Asset-backed securities		16,708		—		(13)		16,695
U.S. government agency debt securities		49,511		—		(89)		49,422
Total marketable securities	$	188,822	$	—	$	(170)	$	188,652


	December 31, 2016
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gain		Loss		Value
Marketable securities:
Corporate debt securities	$	51,352	$	—	$	(59)	$	51,293
Commercial paper		20,463		—		—		20,463
Asset-backed securities		28,692		6		(1)		28,697
U.S. government agency debt securities		43,505		8		(3)		43,510
Total marketable securities	$	144,012	$	14	$	(63)	$	143,963

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2017

Property and Equipment, Net

Schedule of property and equipment, net


	December 31,
	2017		2016
Computer equipment	$	650	$	310
Manufacturing equipment		511		149
Lab equipment		721		—
Furniture and fixtures		327		115
Leasehold improvements		430		33
Property and equipment, gross		2,639		607
Accumulated depreciation		(480)		(126)
Property and equipment, net	$	2,159	$	481

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Accrued Expenses

Schedule of accrued expenses


	December 31,
	2017		2016
Employee compensation expenses	$	3,010	$	1,732
Research and development expenses		627		1,166
Payable to NST		590		—
Vixen contract payable		100		100
Capital leases, current portion		142		—
Other		471		380
Total accrued expenses	$	4,940	$	3,378

Stock-Based Awards (Tables)

12 Months Ended

Dec. 31, 2017

Stock-Based Awards

Assumptions used to determine fair value of stock options granted


	Year Ended
	December 31,
	2017		2016		2015

Risk-free interest rate	1.93	%	2.06	%	1.78	%
Expected term (in years)	6.2		6.5		6.3
Expected volatility	94.19	%	94.86	%	93.90	%
Expected dividend yield	0	%	0	%	0	%

Summary of stock option activity


					Weighted
			Weighted		Average
			Average		Remaining	Aggregate
	Number		Exercise		Contractual	Intrinsic
	of Shares		Price		Term	Value
					(in years)
Outstanding as of December 31, 2014	500,262		$	1.22	9.77	$	305
Granted	1,238,262			18.08
Exercised	—			—
Forfeited and canceled	—			—
Outstanding as of December 31, 2015	1,738,524			13.23	9.51		24,722
Granted	1,083,919			27.12
Exercised	(51,980)			—
Forfeited and canceled	(68,113)			—
Outstanding as of December 31, 2016	2,702,350		$	18.94	9.05	$	24,434
Granted	790,100			26.21
Exercised	(36,738)			6.40
Forfeited and cancelled	(126,955)			22.05
Outstanding as of December 31, 2017	3,328,757		$	20.69	8.28	$	19,812
Options vested and expected to vest as of December 31, 2017	3,328,757		$	20.69	8.28	$	19,812
Options exercisable as of December 31, 2017	1,239,736	(1)	$	15.56	7.43	$	13,414

(1)

Summary of restricted stock units activity


		Weighted
		Average
		Grant Date
	Number	Fair Value
	of Shares	Per Share
Outstanding as of December 31, 2014	—
Granted	53,800	$	28.68
Vested	—
Forfeited and cancelled	—
Outstanding as of December 31, 2015	53,800		28.68
Granted	180,764		27.16
Vested	(12,950)		28.68
Forfeited and cancelled	(2,000)		28.68
Outstanding as of December 31, 2016	219,614	$	27.43
Granted	117,883		26.27
Vested	(40,705)		26.89
Forfeited and cancelled	(13,239)		27.53
Outstanding as of December 31, 2017	283,553	$	27.02

Stock-based compensation expense


	Year Ended
	December 31,
	2017		2016		2015

Cost of revenue	$	211	$	—	$	—
Research and development		5,471		2,291		257
General and administrative		8,748		3,813		634
Total stock-based compensation expense	$	14,430	$	6,104	$	891

Net Loss per Share (Tables)

12 Months Ended

Dec. 31, 2017

Net Loss per Share

Basic and diluted net loss per share


	Year Ended
	December 31,
	2017		2016		2015
Numerator:
Net loss	$	(68,523)	$	(48,079)	$	(20,563)
Accretion of redeemable convertible preferred stock		—		—		(2,566)
Net loss attributable to common stockholders	$	(68,523)	$	(48,079)	$	(23,129)
Denominator:
Weighted average shares of common stock outstanding		28,102,386		21,415,733		6,637,678
Less: Weighted average shares of unvested restricted common stock outstanding		—		—		(530,636)
Weighted average common shares outstanding used in calculating net loss per share attributable to common stockholders, basic and diluted		28,102,386		21,415,733		6,107,042
Net loss per share attributable to common stockholders, basic and diluted	$	(2.44)	$	(2.25)	$	(3.79)

Potential common shares excluded from the calculation of diluted net loss per share attributable to common stockholders


	Year Ended
	December 31,
	2017	2016	2015

Options to purchase common stock	3,328,757	2,702,350	1,738,524
Restricted stock unit awards	283,553	219,614	53,800
Total potential common shares	3,612,310	2,921,964	1,792,324

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Commitments and Contingencies

Schedule of future minimum lease payments


Year Ending December 31,
2018	$	664
2019		627
2020		589
2021		605
2022		622
Thereafter		532
Total	$	3,639

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Taxes

Schedule of loss before income taxes by jurisdiction


	Year Ended December 31,
	2017		2016		2015
U.S. operations	$	(63,665)	$	(40,597)	$	(11,823)
Foreign operations		(6,688)		(7,482)		(8,740)
Loss before income taxes	$	(70,353)	$	(48,079)	$	(20,563)

Reconciliation of statutory to effective rate


	Year Ended December 31,
	2017		2016		2015
Federal statutory income tax rate	(34.0)	%	(34.0)	%	(34.0)	%
State taxes, net of federal benefit	(9.7)		(5.2)		(3.8)
Research and development tax credits	(1.1)		(2.0)		(1.5)
Permanent differences	0.4		1.8		—
Foreign rate differential	1.7		3.2		6.0
Change in deferred tax asset valuation allowance	17.4		36.2		33.3
Impact of U.S. tax reform	22.7		—		—
Effective income tax rate	(2.6)	%	—	%	—	%

Schedule of Deferred Tax Assets and Liabilities [Table Text Block]


		December 31,
	2017		2016
Deferred tax assets:
Net operating loss carryforwards	$	26,566	$	16,836
Capitalized start up costs		9,940		8,840
Research and development tax credit carryforwards		2,296		1,480
Capitalized research and development expenses		3,595		594
Intangible asset		—		1,403
Stock‑based compensation expenses		6,220		2,629
Property and equipment		86		199
Other		280		2
Total deferred tax assets		48,983		31,983
Deferred tax liabilities:
Section 481(a) adjustment		(498)		(1,257)
Intangible asset		(1,843)		—
Other		(313)		—
Total deferred tax liabilities		(2,654)		(1,257)
Valuation allowance		(46,878)		(30,726)
Deferred tax liabilities, net	$	(549)	$	—

Changes in deferred tax asset valuation allowance


		Year Ended December 31,
		2017		2016		2015
Valuation allowance at beginning of year	$	(30,726)	$	(13,286)	$	(6,444)
Decreases recorded as benefit to income tax provision		—		—		—
Increases resulting from the acquisition of Confluence		(4,176)		—		—
Increases recorded to income tax provision		(11,976)		(17,440)		(6,842)
Valuation allowance as of end of year	$	(46,878)	$	(30,726)	$	(13,286)

Schedule of Changes in Unrecognized Tax Benefits


	Year ended December 31,
	2017		2016		2015
Unrecognized tax benefits at beginning of year	$	—	$	(4,400)	$	—
Increases related to prior year tax provisions		(43)		—		(2,624)
Decreases related to prior year tax provisions		—		4,400		—
Increases related to current year tax provisions		—		—		(1,776)
Unrecognized tax benefits as of end of year	$	(43)	$	—	$	(4,400)

Related Party Transactions (Tables)

12 Months Ended

Dec. 31, 2017

Related Party Transactions


	Year Ended
	December 31,
	2017		2016		2015
Services provided by NST Consulting, LLC	$	225	$	323	$	455
Services provided to NST Consulting, LLC		(17)		(56)		(294)
General and administrative expense, net	$	208	$	267	$	161

Services provided by NST Consulting, LLC	$	—	$	246	$	52
Services provided to NST Consulting, LLC		—		(97)		(259)
Research and development expense, net	$	—	$	149	$	(207)

Services provided by NST Consulting, LLC	$	225	$	569	$	507
Services provided to NST Consulting, LLC		(17)		(153)		(553)
Total, net	$	208	$	416	$	(46)


Net payments made to (received from) NST Consulting, LLC	$	300	$	325	$	(46)

Segment Information (Tables)

12 Months Ended

Dec. 31, 2017

Segment Reporting [Abstract]

Schedule of Segment Reporting Information, by Segment [Table Text Block]


	Dermatology		Contract		Corporate		Total
Year Ended December 31, 2017	Therapeutics		Research		and Other		Company
Revenue	$	—	$	3,202	$	(1,519)	$	1,683
Cost of revenue		—		2,726		(1,519)		1,207
Research and development		39,790		—		—		39,790
General and administrative		13,991		673		18,445		33,109
Loss from operations	$	(53,781)	$	(197)	$	(18,445)	$	(72,423)


	Dermatology		Contract		Corporate		Total
Year Ended December 31, 2016	Therapeutics		Research		and Other		Company
Revenue	$	—	$	—	$	—	$	—
Cost of revenue		—		—		—		—
Research and development		33,476		—		—		33,476
General and administrative		3,450		—		11,641		15,091
Loss from operations	$	(36,926)	$	—	$	(11,641)	$	(48,567)


	Dermatology		Contract		Corporate		Total
Year Ended December 31, 2015	Therapeutics		Research		and Other		Company
Revenue	$	—	$	—	$	—	$	—
Cost of revenue		—		—		—		—
Research and development		15,339		—		—		15,339
General and administrative		1,195		—		4,133		5,328
Loss from operations	$	(16,534)	$	—	$	(4,133)	$	(20,667)

Quarterly Financial Information (unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Information [Table Text Block]


	2017 Quarter Ended
	March 31,		June 30,		September 30,		December 31,
Revenue	$	—	$	—	$	684	$	999
Gross profit		—		—		231		245
Operating expenses		12,930		15,295		18,987		25,687
Other income, net		371		457		564		678
Net loss		(12,559)		(14,838)		(18,192)		(22,934)
Net loss per share, basic and diluted	$	(0.48)	$	(0.56)	$	(0.63)	$	(0.74)

	2016 Quarter Ended
	March 31,		June 30,		September 30,		December 31,
Revenue	$	—	$	—	$	—	$	—
Gross profit		—		—		—		—
Operating expenses		13,139		12,989		10,812		11,627
Other income, net		100		118		118		152
Net loss		(13,039)		(12,871)		(10,694)		(11,475)
Net loss per share, basic and diluted	$	(0.65)	$	(0.62)	$	(0.50)	$	(0.49)

Organization and Nature of Business (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization and Nature of Business
Cash, cash equivalents and marketable securities	$ 208,854
Accumulated deficit	$ 159,435	$ 90,912

Summary of Significant Accounting Policies (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary of Significant Accounting Policies
Number of active research grants \| item		2
Expected dividend yield (as a percent)		0.00%	0.00%	0.00%
Reverse stock split	0.2899
Write-down of property and equipment held for sale			$ 216	$ 289
Assets held for sale		$ 0	$ 0
PDUFA fee paid for which waiver has been applied		$ 2,000
Number of financial institutions holding entity funds \| item		1

Summary of Significant Accounting Policies - Risk (Details) - Laboratory servicer revenues - Customer Concentration Risk [Member]	12 Months Ended
	Dec. 31, 2017 customer
Concentration Risk [Line Items]
Number of customers comprising risk percentage	5
Concentration of risk (as a percent)	70.00%

Summary of Significant Accounting Policies - Deferred costs, etc. (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) item	Aug. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Accounting Policies [Line Items]
Deferred offering costs	$ 62		$ 116
Goodwill	$ 18,504
Number of operating segments \| item	2
Confluence
Summary Of Accounting Policies [Line Items]
Goodwill		$ 18,504
Computer equipment
Summary Of Accounting Policies [Line Items]
Depreciation period (in years)	3 years
Manufacturing equipment
Summary Of Accounting Policies [Line Items]
Depreciation period (in years)	5 years
Furniture and Fixtures
Summary Of Accounting Policies [Line Items]
Depreciation period (in years)	5 years

Confluence Acquisition - Summary of fair value consideration (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2017	Dec. 31, 2017
Fair value of consideration
Cash consideration paid		$ 9,647
Confluence
Business Acquisition [Line Items]
Acquisition of outstanding common shares (as a percent)	100.00%
Additional contingent consideration based on milestones, maximum, per Agreement		80,000
Additional contingent consideration based on milestones, portion payable in shares		$ 2,500
Royalty term		10 years
Fair value of consideration
Cash consideration paid	$ 10,269
Aclaris common stock issued	9,675
Contingent consideration	4,378
Total fair value of consideration to Confluence equity holders	$ 24,322

Confluence Acquisition - Fair value of assets acquired and liabilities assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Aug. 31, 2017
Business Acquisition [Line Items]
Goodwill	$ 18,504
Confluence
Business Acquisition [Line Items]
Cash and cash equivalents		$ 622
Accounts receivable, net		574
Other current assets		89
Property and equipment		268
Other intangible assets		751
IPR&D		6,629
Goodwill		18,504
Total assets acquired		27,437
Accounts payable and accrued expenses		656
Deferred tax liability		2,386
Other liabilities		73
Total liabilities assumed		3,115
Total net assets acquired		$ 24,322

Confluence Acquisition - Supplemental pro forma financial information (Details) - Confluence - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Revenue	$ 4,365	$ 3,693	$ 2,630
Gross profit	1,347	1,652	1,302
Total operating expenses	73,810	51,277	24,151
Net loss	(70,391)	(49,148)	(22,803)
Acquisition-related expenses
Business Acquisition [Line Items]
Net loss	1,351	0	0
Billed revenues
Business Acquisition [Line Items]
Net loss	$ 888	$ 0	$ 0

Fair Value of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 188,652	$ 143,963
Liabilities:
Transfers from Level 1 to Level 2	0	0
Transfers from Level 2 to Level 1	0	0
Transfers into or out of Level 3	0	0
Recurring
Assets:
Cash equivalents	19,339	24,213
Marketable securities	188,652	143,963
Total assets measured at fair value	207,991	168,176
Liabilities:
Acquisition-related contingent consideration	4,378
Total liabilities measured at fair value	4,378
Recurring \| Level 1
Assets:
Cash equivalents	19,339	11,522
Total assets measured at fair value	19,339	11,522
Recurring \| Level 2
Assets:
Cash equivalents		12,691
Marketable securities	188,652	143,963
Total assets measured at fair value	188,652	$ 156,654
Recurring \| Level 3
Liabilities:
Acquisition-related contingent consideration	4,378
Total liabilities measured at fair value	$ 4,378

Fair Value of Financial Assets and Liabilities - by type (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Marketable securities:
Amortized Cost	$ 188,822	$ 144,012
Gross Unrealized Gain		14
Gross Unrealized Loss	(170)	(63)
Fair Value	188,652	143,963
Corporate debt securities
Marketable securities:
Amortized Cost	37,401	51,352
Gross Unrealized Loss	(68)	(59)
Fair Value	37,333	51,293
Commercial paper
Marketable securities:
Amortized Cost	85,202	20,463
Fair Value	85,202	20,463
Asset-backed securities
Marketable securities:
Amortized Cost	16,708	28,692
Gross Unrealized Gain		6
Gross Unrealized Loss	(13)	(1)
Fair Value	16,695	28,697
U.S. government agency debt securities
Marketable securities:
Amortized Cost	49,511	43,505
Gross Unrealized Gain		8
Gross Unrealized Loss	(89)	(3)
Fair Value	$ 49,422	$ 43,510

- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References + Details

Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property and Equipment, net
Property and equipment	$ 2,639	$ 607
Accumulated depreciation	(480)	(126)
Property and equipment, net	2,159	481
Depreciation	370	120	$ 90
Computer equipment
Property and Equipment, net
Property and equipment	650	310
Manufacturing equipment
Property and Equipment, net
Property and equipment	511	149
Laboratory Equipment [Member]
Property and Equipment, net
Property and equipment	721
Furniture and Fixtures
Property and Equipment, net
Property and equipment	327	115
Leasehold Improvements
Property and Equipment, net
Property and equipment	$ 430	$ 33

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Expenses
Employee compensation expenses	$ 3,010	$ 1,732
Research and development expenses	627	1,166
Payable to NST	590
Vixen contract payable.	100	100
Capital leases, current portion	142
Other	471	380
Total accrued expenses	$ 4,940	$ 3,378

Stockholders' Equity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Vote $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($)	Oct. 31, 2015 shares	Aug. 28, 2015 shares
Stockholders' Equity
Preferred stock, shares authorized	10,000,000	10,000,000		10,000,000
Preferred stock, shares outstanding	0	0
Convertible preferred stock, shares outstanding					11,677,076
Common stock shares authorized	100,000,000	100,000,000
Common stock par value (in dollars per share) \| $ / shares	$ 0.00001	$ 0.00001
Number of votes per share \| Vote	1
Dividends declared \| $	$ 0	$ 0	$ 0

Stockholders' Equity - Restricted Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based awards
Number of shares of restricted stock issued	2,730,427
Number of shares subject to vesting	1,918,834
Granted, estimated grant-date fair value (in dollars per share)	$ 0.00001
Aggregate fair value of restricted stock awards vested during the year		$ 0	$ 0	$ 6,423
Restricted common stock \| Tranche One
Stock-based awards
Percentage of stock subject to vesting (as a percent)	25.00%
Vesting period (in years)	1 year
Restricted common stock \| Tranche Two
Stock-based awards
Percentage of stock subject to vesting (as a percent)	75.00%
Vesting period (in years)	3 years

Stockholders' Equity - IPO, etc. (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended		12 Months Ended
	Nov. 23, 2016	Nov. 17, 2016	Oct. 13, 2015	Aug. 31, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsidiary, Sale of Stock [Line Items]
Number of shares issued				3,747,602
Issuance price (in dollars per share)				$ 23.02
Gross proceeds				$ 86,270
Underwriters' discounts and commissions				5,176
Expenses related to stock issuance				176
Offering proceeds, net of discounts, commissions and expenses				$ 80,918
Common Stock
Subsidiary, Sale of Stock [Line Items]
Number of shares issued								5,750,000
Issuance of stock upon conversion, shares			11,677,076					11,677,076
IPO
Subsidiary, Sale of Stock [Line Items]
Number of shares issued			5,000,000
Issuance price (in dollars per share)			$ 11.00
Gross proceeds			$ 55,000
Underwriters' discounts and commissions			4,428
Expenses related to stock issuance			2,272
Offering proceeds, net of discounts, commissions and expenses			$ 56,550
Exercise of over-allotment option
Subsidiary, Sale of Stock [Line Items]
Number of shares issued		600,000	750,000
Issuance price (in dollars per share)		$ 22.75	$ 11.00
Gross proceeds		$ 13,650	$ 8,250
Private Placement
Subsidiary, Sale of Stock [Line Items]
Number of shares issued					1,081,082
Issuance price (in dollars per share)					$ 18.50
Gross proceeds					$ 20,000
Expenses related to stock issuance					153
Placement fees					1,300
Offering proceeds, net of discounts, commissions and expenses					$ 18,547
Private Placement \| Common Stock
Subsidiary, Sale of Stock [Line Items]
Number of shares issued							1,081,082
Follow On Public Offering
Subsidiary, Sale of Stock [Line Items]
Number of shares issued	4,000,000
Issuance price (in dollars per share)	$ 22.75
Gross proceeds	$ 91,000
Underwriters' discounts and commissions	6,279
Expenses related to stock issuance	188
Offering proceeds, net of discounts, commissions and expenses	$ 98,158
Follow On Public Offering \| Common Stock
Subsidiary, Sale of Stock [Line Items]
Number of shares issued						3,747,602	4,600,000
At The Market Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Number of shares issued						635,000
Issuance price (in dollars per share)						$ 31.50
Gross proceeds						$ 20,003
Expenses related to stock issuance						$ 691
At The Market Offering [Member] \| Common Stock
Subsidiary, Sale of Stock [Line Items]
Number of shares issued						635,000

Stock-Based Awards (Details) - shares		12 Months Ended			44 Months Ended
Stock-Based Awards (Details) - shares	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 14, 2015	Jul. 31, 2017	Sep. 30, 2015	Dec. 31, 2014
Stock-based awards
Options granted (in shares)		790,100	1,083,919	1,238,262
Options outstanding		3,328,757	2,702,350	1,738,524				500,262
Stock Option Valuation
Risk-free interest rate (as a percent)		1.93%	2.06%	1.78%
Expected term (in years)		6 years 2 months 12 days	6 years 6 months	6 years 3 months 18 days
Expected volatility (as a percent)		94.19%	94.86%	93.90%
Expected dividend yield (as a percent)		0.00%	0.00%	0.00%
2017 Inducement Plan
Stock-based awards
Number of shares authorized						1,000,000
Number of shares available for grant						489,884
2015 Equity Incentive Plan
Stock-based awards
Number of shares authorized							1,643,872
Number of shares available for grant		1,404,498
Percentage increase to shares available for grant from common outstanding as of preceding December 31 (as a percent)		4.00%
Additional shares available	1,234,260
2012 Equity Compensation Plan
Stock-based awards
Number of shares available for grant		0
Options granted (in shares)					1,140,524
Options outstanding		984,720	1,049,667
Vesting period (in years)		4 years
Term of award (in years)		10 years

Stock-Based Awards - Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options, Number of Shares
Number of Shares, beginning balance	2,702,350	1,738,524	500,262
Number of Shares, Granted	790,100	1,083,919	1,238,262
Number of Shares, Exercised	(36,738)	(51,980)
Number of Shares, Forfeited and cancelled	(126,955)	(68,113)
Number of Shares, ending balance	3,328,757	2,702,350	1,738,524	500,262
Number of Shares, Options vested and expected to vest	3,328,757
Number of Shares, Options exercisable	1,239,736
Options, Weighted Average Exercise Price
Weighted Average Exercise Price, beginning balance (in dollars per share)	$ 18.94	$ 13.23	$ 1.22
Weighted Average Exercise Price, Granted (in dollars per share)	26.21	27.12	18.08
Weighted Average Exercise Price, Exercised (in dollars per share)	6.40
Weighted Average Exercise Price, Forfeited and cancelled (in dollars per share)	22.05
Weighted Average Exercise Price, ending balance (in dollars per share)	20.69	$ 18.94	$ 13.23	$ 1.22
Weighted Average Exercise Price, Options vested and expected to vest (in dollars per share)	20.69
Weighted Average Exercise Price, Options exercisable (in dollars per share)	$ 15.56
Options, Weighted Average Remaining Contractual Term
Weighted Average Remaining Contractual Term (in years)	8 years 3 months 11 days	9 years 18 days	9 years 6 months 4 days	9 years 9 months 7 days
Weighted Average Remaining Contractual Term, Options vested and expected to vest (in years)	8 years 3 months 11 days
Weighted Average Remaining Contractual Term, Options exercisable (in years)	7 years 5 months 5 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value	$ 19,812	$ 24,434	$ 24,722	$ 305
Aggregate Intrinsic Value, Options vested and expected to vest	19,812
Aggregate Intrinsic Value, Options exercisable	$ 13,414
Weighted average grant-date fair value of stock options granted (in dollars per share)	$ 20.28	$ 21.16	$ 13.84

Stock-Based Awards - RSUs (Details) - $ / shares	1 Months Ended	12 Months Ended
Stock-Based Awards - RSUs (Details) - $ / shares	Jul. 31, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
RSU, Weighted Average Grant Date Fair Value Per Unit
Granted, estimated grant-date fair value (in dollars per share)	$ 0.00001
Restricted Stock Unit Awards
RSU, Number of Units
Units outstanding, beginning of period		219,614	53,800
Granted		117,883	180,764	53,800
Vested		(40,705)	(12,950)
Forfeited and cancelled		(13,239)	(2,000)
Units outstanding, end of period		283,553	219,614	53,800
RSU, Weighted Average Grant Date Fair Value Per Unit
Weighted average grant date fair value, beginning balance (in dollars per share)		$ 27.43	$ 28.68
Granted, estimated grant-date fair value (in dollars per share)		26.27	27.16	$ 28.68
Weighted average grant date fair value, vested (in dollars per share)		26.89	28.68
Forfeited and cancelled, estimated grant date fair value (in dollars per share)		27.53	28.68
Weighted average grant date fair value, ending balance (in dollars per share)		$ 27.02	$ 27.43	$ 28.68

Stock-Based Awards - Compensation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation expense
Stock-based compensation expense	$ 14,430	$ 6,104	$ 891
Unrecognized stock-based compensation cost, options	36,150
Unrecognized compensation, RSUs	5,849
Cost of revenue.
Stock-based compensation expense
Stock-based compensation expense	211
Research and development expense.
Stock-based compensation expense
Stock-based compensation expense	5,471	2,291	257
General and administrative expense.
Stock-based compensation expense
Stock-based compensation expense	$ 8,748	$ 3,813	$ 634
Options to Purchase Common Stock
Stock-based compensation expense
Weighted average recognition period unrecognized stock-based compensation cost (in years)	2 years 11 months 5 days
Restricted Stock Unit Awards
Stock-based compensation expense
Weighted average recognition period unrecognized stock-based compensation cost (in years)	2 years 11 months 5 days

Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Operating loss									$ (68,523)	$ (48,079)	$ (20,563)
Accretion of convertible preferred stock											(2,566)
Net loss attributable to common stockholders	$ (22,934)	$ (18,192)	$ (14,838)	$ (12,559)	$ (11,475)	$ (10,694)	$ (12,871)	$ (13,039)	$ (68,523)	$ (48,079)	$ (23,129)
Denominator:
Weighted average shares of common stock outstanding (in shares)									28,102,386	21,415,733	6,637,678
Less: Weighted average shares of unvested restricted common stock outstanding (in shares)											(530,636)
Weighted average shares of common stock outstanding									28,102,386	21,415,733	6,107,042
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.74)	$ (0.63)	$ (0.56)	$ (0.48)	$ (0.49)	$ (0.50)	$ (0.62)	$ (0.65)	$ (2.44)	$ (2.25)	$ (3.79)

Net Loss per Share - Anti-dilution (Details) - shares	12 Months Ended
Net Loss per Share - Anti-dilution (Details) - shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from the calculation of diluted net loss per share attributable to common stockholders	3,612,310	2,921,964	1,792,324
Options to Purchase Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from the calculation of diluted net loss per share attributable to common stockholders	3,328,757	2,702,350	1,738,524
Restricted Stock Unit Awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common shares excluded from the calculation of diluted net loss per share attributable to common stockholders	283,553	219,614	53,800

Commitments and Contingencies (Details) $ in Thousands	12 Months Ended
Commitments and Contingencies (Details) $ in Thousands	Dec. 31, 2017 USD ($) ft² item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Agreements for Office Space
Area leased from third party \| ft²	33,019
Rent expense under operating leases	$ 946	$ 254	$ 119
Future minimum lease payments under the sublease
2018	664
2019	627
2020	589
2021	605
2022	622
Thereafter	532
Total	$ 3,639
Number of capital lease arrangements \| item	2
Stock Purchase Agreement [Member]
Supply Agreement
Amount of required annual payment under the contract	$ 100

Income Taxes - Tax Cuts and Jobs Act (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect Of Tax Cuts And Jobs Act Of 2017 [Abstract]
Date accounting was completed			Dec. 31, 2017
Federal statutory income tax rate	35.00%		34.00%	34.00%	34.00%
Tax expense attributable to revaluation of deferred tax liabilities, net			$ 18,507
Reduction in valuation allowance			20,344
Tax benefit attributable to revaluation of deferred tax liabilities, net, after effect on valuation allowance			1,837
Tax impact of one-time transition tax			$ 0
Scenario, Forecast [Member]
Effect Of Tax Cuts And Jobs Act Of 2017 [Abstract]
Federal statutory income tax rate		21.00%

Income Taxes - Rate reconciliation (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Federal income tax benefit		$ 0	$ 0	$ 0
State income tax benefit		0	0	0
Loss before income taxes
U.S. operations		(63,665)	(40,597)	(11,823)
Foreign operations		(6,688)	(7,482)	(8,740)
Loss before income taxes		$ (70,353)	$ (48,079)	$ (20,563)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory income tax rate	(35.00%)	(34.00%)	(34.00%)	(34.00%)
State taxes, net of federal benefit		(9.70%)	(5.20%)	(3.80%)
Research and development tax credits		(1.10%)	(2.00%)	(1.50%)
Permanent differences		0.40%	1.80%
Foreign rate differential		1.70%	3.20%	6.00%
Change in deferred tax asset valuation allowance		17.40%	36.20%	33.30%
Impact of U.S. tax reform		22.70%
Effective income tax rate		(2.60%)

Income Taxes - Deferred Assets and Liabilities, CFDs (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of net deferred tax assets
Net operating loss carryforwards	$ 26,566	$ 16,836
Capitalized start-up costs	9,940	8,840
Research and development tax credit carryforwards	2,296	1,480
Capitalized research and development expenses	3,595	594
Intangible assets		1,403
Stock‑based compensation expenses	6,220	2,629
Property and equipment	86	199
Other	280	2
Total deferred tax assets	48,983	31,983
Section 481 (a) adjustment	(498)	(1,257)
Intangible asset	(1,843)
Other	(313)
Total deferred tax liabilities	(2,654)	(1,257)
Valuation allowance	(46,878)	$ (30,726)	$ (13,286)	$ (6,444)
Deferred Tax Liabilities, Net, Total	(549)
United Kingdom Tax Authority
Components of net deferred tax assets
Operating Loss Carryforwards	1,292
Federal
Components of net deferred tax assets
Operating Loss Carryforwards	76,310
Federal \| Research Tax Credit Carryforward [Member]
Components of net deferred tax assets
Tax Credit Carryforward, Amount	2,202
State
Components of net deferred tax assets
Operating Loss Carryforwards	117,808
State \| Research Tax Credit Carryforward [Member]
Components of net deferred tax assets
Tax Credit Carryforward, Amount	$ 118

Income Taxes - Valuation Allowance and Unrecognized Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in valuation allowance for deferred tax assets
Valuation allowance at beginning of year	$ (30,726)	$ (13,286)	$ (6,444)
Increases resulting from the acquistion of Confluence	(4,176)
Increases recorded to income tax provision	(11,976)	(17,440)	(6,842)
Valuation allowance as of end of year	(46,878)	(30,726)	(13,286)
Unrecognized Tax Benefits
Unrecognized Tax Benefits, Beginning Balance		(4,400)
Increases related to prior year tax provisions	(43)		(2,624)
Decreases related to prior year tax provisions		4,400
Increases related to current year tax provisions			(1,776)
Unrecognized Tax Benefits, Ending Balance	(43)		(4,400)
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	36	0
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ 3	$ 0	$ 0

Related Party Transactions (Details) $ in Thousands	12 Months Ended
Related Party Transactions (Details) $ in Thousands	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Nov. 30, 2017 USD ($)
NST, LLC [Member]
Related Party Transactions
Number of executive officers affiliated with counterparty \| item	3
Board of Directors Chairman [Member] \| Direct sublease agreement
Related Party Transactions
Amount due to related party				$ 590
Lease termination charge	$ 506
Expenses incurred under related party transactions	318	$ 253	$ 127
Board of Directors Chairman [Member] \| Services agreement
Related Party Transactions
Amount due to related party	570	91
Expenses incurred under related party transactions	225	569	507
Other revenue earned from related party transactions	(17)	(153)	(553)
Net expenses (revenues) from related party transactions	208	416	(46)
Payments to related party	300	325
Receipts from related party			(46)
Board of Directors Chairman [Member] \| Services agreement \| General and administrative expense.
Related Party Transactions
Expenses incurred under related party transactions	225	323	455
Other revenue earned from related party transactions	(17)	(56)	(294)
Net expenses (revenues) from related party transactions	$ 208	267	161
Board of Directors Chairman [Member] \| Services agreement \| Research and development expense.
Related Party Transactions
Expenses incurred under related party transactions		246	52
Other revenue earned from related party transactions		(97)	(259)
Net expenses (revenues) from related party transactions		$ 149	$ (207)

Agreements Related to Intellectual Property (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) item shares	Aug. 31, 2012	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)
Assignment Agreement and Finder's Services Agreement [Member] \| A-101 40% Topical Solution
Agreements Related to Intellectual Property
Number of products \| item			1
Term of agreement, minimum		15 years
Assignment Agreement and Finder's Services Agreement [Member] \| Achievement Of Dosing Of First Subject With A101 \| A-101 40% Topical Solution
Agreements Related to Intellectual Property
Milestone payment						$ 400
Assignment Agreement and Finder's Services Agreement [Member] \| Achievement Of Specified Regulatory Milestones [Member] \| A-101 40% Topical Solution
Agreements Related to Intellectual Property
Potential future milestone payments			$ 0
Finders Services Agreement \| Achievement Of Dosing Of First Subject With A101 \| A-101 40% Topical Solution
Agreements Related to Intellectual Property
Milestone payment				$ 300
Finders Services Agreement \| Achievement Of Specified Commercial Milestones \| A-101 40% Topical Solution
Agreements Related to Intellectual Property
Potential future milestone payments			4,500
Finders Services Agreement \| Achievement Of Specified Regulatory Milestones [Member] \| A-101 40% Topical Solution
Agreements Related to Intellectual Property
Milestone payment			$ 1,000
License and collaboration agreement \| JAK inhibitors
Agreements Related to Intellectual Property
Upfront payment made					$ 8,000
Period from first commercial product sale that royalties are owed (in years)			10 years
License and collaboration agreement \| Achievement Of Pre Commercialization Milestones \| JAK inhibitors
Agreements Related to Intellectual Property
Potential future milestone payments			$ 80,000
License and collaboration agreement \| Achievement Of Development Milestones [Member] \| JAK inhibitors
Agreements Related to Intellectual Property
Potential future milestone payments			$ 10,000
Stock Purchase Agreement [Member]
Agreements Related to Intellectual Property
Term of agreement, minimum			10 years
Upfront payment made	$ 600
Entity stock issued in stock purchase \| shares	159,420
Amount of required annual payment under the contract			$ 100
Fair value of common stock issued				$ 2,355
Number of annual payments \| item				6
Stock Purchase Agreement [Member] \| Achievement Of Specified Commercial Milestones
Agreements Related to Intellectual Property
Potential future milestone payments	$ 22,500
Stock Purchase Agreement [Member] \| Achievement Of Pre Commercialization Milestones
Agreements Related to Intellectual Property
Number of products \| item	3
Potential future milestone payments	$ 18,000
License Agreement
Agreements Related to Intellectual Property
Term of agreement, minimum			10 years
Annual maintenance fee			$ 10
License Agreement \| Achievement Of Specified Commercial Milestones
Agreements Related to Intellectual Property
Potential future milestone payments			$ 11,600

401(k) Savings Plan (Details) - USD ($) $ in Thousands	12 Months Ended
401(k) Savings Plan (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan [Abstract]
Employer match of employee contributions (as a percent)	100.00%
Employee earnings subject to employer match (as a percent)	4.00%
Company contributions	$ 270	$ 176	$ 99

Segment Information (Details) $ in Thousands	12 Months Ended
Segment Information (Details) $ in Thousands	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| item	2
Revenues	$ 1,683
Cost of revenue	1,207
Research and development	39,790	$ 33,476	$ 15,339
General and administrative	33,109	15,091	5,328
Loss from operations	(72,423)	(48,567)	(20,667)
UNITED KINGDOM
Segment Reporting Information [Line Items]
Net assets	175	4,786
Dermatology Therapeutics Segment [Member]
Segment Reporting Information [Line Items]
Research and development	39,790	33,476	15,339
General and administrative	13,991	3,450	1,195
Loss from operations	(53,781)	(36,926)	(16,534)
Contract Research Segment [Member]
Segment Reporting Information [Line Items]
Revenues	3,202
Cost of revenue	2,726
General and administrative	673
Loss from operations	(197)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues	(1,519)
Cost of revenue	(1,519)
General and administrative	18,445	11,641	4,133
Loss from operations	$ (18,445)	$ (11,641)	$ (4,133)

Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 999	$ 684							$ 1,683
Gross Profit	245	231							476
Operating Expenses	25,687	18,987	$ 15,295	$ 12,930	$ 11,627	$ 10,812	$ 12,989	$ 13,139	72,899	$ 48,567	$ 20,667
Other income, net	678	564	457	371	152	118	118	100
Operating loss									(68,523)	(48,079)	(20,563)
Net loss	$ (22,934)	$ (18,192)	$ (14,838)	$ (12,559)	$ (11,475)	$ (10,694)	$ (12,871)	$ (13,039)	$ (68,523)	$ (48,079)	$ (23,129)
Net loss per share, basic and diluted	$ (0.74)	$ (0.63)	$ (0.56)	$ (0.48)	$ (0.49)	$ (0.50)	$ (0.62)	$ (0.65)	$ (2.44)	$ (2.25)	$ (3.79)