Flexion Therapeutics (FLXN) 10-Q 6 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 31, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	FLXN
Entity Registrant Name	Flexion Therapeutics Inc
Entity Central Index Key	1,419,600
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		37,540,829

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets (Unaudited) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 159,179,000	$ 30,915,000
Marketable securities	175,921,000	174,688,000
Prepaid expenses and other current assets	3,609,000	3,790,000
Total current assets	338,709,000	209,393,000
Property and equipment, net	11,481,000	11,664,000
Long-term investments		4,725,000
Restricted cash	600,000	480,000
Total assets	350,790,000	226,262,000
Current liabilities:
Accounts payable	3,960,000	2,161,000
Accrued expenses and other current liabilities	12,276,000	6,245,000
Current portion of long-term debt	9,967,000	9,134,000
Total current liabilities	26,203,000	17,540,000
Long-term debt, net	15,260,000	21,399,000
2024 convertible notes, net	135,275,000
Other long-term liabilities	399,000	291,000
Total liabilities	177,137,000	39,230,000
Commitments and contingencies
Preferred stock, $0.001 par value; 10,000,000 shares authorized at September 30, 2017 and December 31, 2016 and 0 shares issued and outstanding at September 30, 2017 and December 31, 2016
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized; 31,983,903 and 31,667,469 shares issued and outstanding, at September 30, 2017 and December 31, 2016, respectively	32,000	32,000
Additional paid-in capital	472,322,000	398,757,000
Accumulated other comprehensive loss	(62,000)	(71,000)
Accumulated deficit	(298,639,000)	(211,686,000)
Total stockholders’ equity	173,653,000	187,032,000
Total liabilities and stockholders’ equity	$ 350,790,000	$ 226,262,000

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares	Sep. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
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	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	31,983,903	31,667,469
Common stock, shares outstanding	31,983,903	31,667,469

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
Research and development	$ 12,846	$ 9,047	$ 35,371	$ 29,933
General and administrative	18,375	8,388	46,533	18,295
Total operating expenses	31,221	17,435	81,904	48,228
Loss from operations	(31,221)	(17,435)	(81,904)	(48,228)
Other income (expense):
Interest income	1,095	421	2,450	1,052
Interest expense	(3,843)	(561)	(7,363)	(1,039)
Other expense	(219)	(207)	(136)	(567)
Total other income (expense)	(2,967)	(347)	(5,049)	(554)
Net loss	$ (34,188)	$ (17,782)	$ (86,953)	$ (48,782)
Net loss per share basic and diluted	$ (1.07)	$ (0.65)	$ (2.73)	$ (2.04)
Weighted average common shares outstanding, basic and diluted	31,931	27,524	31,821	23,938
Other comprehensive income (loss):
Unrealized gains (losses) from available-for-sale securities, net of tax of $0	$ 18	$ 38	$ 9	$ (70)
Total other comprehensive income (loss)	18	38	9	(70)
Comprehensive loss	$ (34,170)	$ (17,744)	$ (86,944)	$ (48,852)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Unrealized gains (losses) from available-for-sale securities, tax	$ 0	$ 0	$ 0	$ 0

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Changes in Stockholder's Equity (Deficit) (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in-Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balance at Dec. 31, 2014	$ 144,941	$ 21	$ 238,402	$ (5)	$ (93,477)
Balance (in shares) at Dec. 31, 2014		21,440
Exercise of stock options	593	$ 1	592
Exercise of stock options (in shares)		109
Employee Stock Purchase Plan	276		276
Employee Stock Purchase Plan (in shares)		21
Stock-based compensation expense	4,583		4,583
Net loss	(46,315)				(46,315)
Other comprehensive loss	(92)			(92)
Balance at Dec. 31, 2015	103,986	$ 22	243,853	(97)	(139,792)
Balance (in shares) at Dec. 31, 2015		21,570
Issuance of Common Stock net of issuance costs	147,501	$ 10	147,491
Issuance of Common Stock net of issuance costs (in shares)		10,040
Exercise of stock options	167		167
Exercise of stock options (in shares)		30
Employee Stock Purchase Plan	476		476
Employee Stock Purchase Plan (in shares)		27
Stock-based compensation expense	6,770		6,770
Net loss	(71,894)				(71,894)
Other comprehensive loss	26			26
Balance at Dec. 31, 2016	187,032	$ 32	398,757	(71)	(211,686)
Balance (in shares) at Dec. 31, 2016		31,667
Exercise of stock options	$ 3,076		3,076
Exercise of stock options (in shares)	261	261
Employee Stock Purchase Plan	$ 453		453
Employee Stock Purchase Plan (in shares)		56
Stock-based compensation expense	7,570		7,570
Convertible debt	62,466		62,466
Net loss	(86,953)				(86,953)
Other comprehensive loss	9			9
Balance at Sep. 30, 2017	$ 173,653	$ 32	$ 472,322	$ (62)	$ (298,639)
Balance (in shares) at Sep. 30, 2017		31,984

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities
Net loss	$ (86,953,000)	$ (48,782,000)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	1,456,000	700,000
Stock-based compensation expense	7,570,000	4,963,000
Amortization of premium (discount) on marketable securities	355,000	544,000
Loss on disposal of fixed assets		2,278,000
Amortization of convertible debt discount and debt issuance costs	2,985,000	26,000
Premium paid on securities purchased	(676,000)	(273,000)
Changes in operating assets and liabilities:
Accounts receivable		95,000
Prepaid expenses, other current and long-term assets	181,000	(697,000)
Accounts payable	2,020,000	(1,029,000)
Accrued expenses and other current and long-term liabilities	7,124,000	690,000
Net cash used in operating activities	(65,938,000)	(41,485,000)
Cash flows from investing activities
Purchases of property and equipment	(1,882,000)	(8,165,000)
Change in restricted cash	(120,000)
Purchases of marketable securities	(199,756,000)	(80,134,000)
Sale and redemption of marketable securities	203,578,000	40,897,000
Net cash provided by investing activities	1,820,000	(47,402,000)
Cash flows from financing activities
Proceeds from the issuance of 2024 convertible notes	201,250,000
Payment of debt issuance costs	(6,470,000)	(42,000)
Proceeds from issuance of notes payable		15,000,000
Proceeds from the offering of common stock		77,644,000
Payments on notes payable	(5,833,000)
Payments of public offering costs	(95,000)	(256,000)
Proceeds from the exercise of stock options	3,077,000	166,000
Proceeds from Employee Stock Purchase Plan	453,000	240,000
Net cash provided by financing activities	192,382,000	92,752,000
Net increase in cash and cash equivalents	128,264,000	3,865,000
Cash and cash equivalents at beginning of period	30,915,000	62,944,000
Cash and cash equivalents at end of period	159,179,000	66,809,000
Supplemental disclosures of cash flow information:
Cash paid for interest	1,334,000	$ 823,000
Supplemental disclosures of non-cash financing activities:
Purchases of property and equipment in accounts payable and accrued expenses	$ 14,000

Overview and Nature of the Busi

Overview and Nature of the Business	9 Months Ended
	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Overview and Nature of the Business	1. Overview and Nature of the Business Flexion Therapeutics, Inc. (“Flexion” or the “Company”) was incorporated under the laws of the state of Delaware on November 5, 2007. Flexion is a specialty pharmaceutical company focused on the development and commercialization of novel, local therapies for the treatment of patients with musculoskeletal conditions, beginning with osteoarthritis (“OA”), a type of degenerative arthritis. On October 6, 2017, the U.S. Food and Drug Administration, or FDA, approved Zilretta TM The Company is subject to risks and uncertainties common to companies in the biopharmaceutical industry, including, but not limited to, new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, and the ability to secure additional capital to fund operations. Successfully commercializing Zilretta will require significant sales and marketing efforts and the Company’s pipeline programs may require significant additional research and development efforts, including extensive preclinical and clinical testing. These activities will in turn require significant amounts of capital, adequate personnel infrastructure and extensive compliance reporting capabilities. There can be no assurance when, if ever, the Company will realize significant revenue from the sales of Zilretta or if the development efforts supporting the Company’s pipeline, including future clinical trials, will be successful.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed consolidated financial statements as of September 30, 2017, and for the three and nine months ended September 30, 2017 and 2016, have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and Generally Accepted Accounting Principles (“GAAP”) for consolidated financial information including the accounts of the Company and its wholly-owned subsidiary after elimination of all significant intercompany accounts and transactions. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, these condensed consolidated financial statements reflect all adjustments which are necessary for a fair statement of the Company’s financial position and results of its operations, as of and for the periods presented. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K filed with the SEC on March 10, 2017. The information presented in the condensed consolidated financial statements and related notes as of September 30, 2017, and for the three and nine months ended September 30, 2017 and 2016, is unaudited. The December 31, 2016 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by GAAP for complete financial statements. Interim results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2017, or any future period. The accompanying condensed consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. The Company has incurred recurring losses and negative cash flows from operations. As of September 30, 2017, the Company had cash, cash equivalents, marketable securities, and long-term investments of approximately $335,100,000. Management believes that current cash, cash equivalents and marketable securities on hand at September 30, 2017, together with the net proceeds of its October 2017 common stock offering of approximately $132,400,000 described in note 12, should be sufficient to fund operations for at least the next twelve months from the issuance date of these financial statements. The future viability of the Company is dependent on sales revenue from Zilretta and its ability to raise additional capital to finance its operations, to fund increased research and development costs in order to seek approval for commercialization of its product candidates, and to successfully commercialize Zilretta. The Company’s failure to raise capital as and when needed would have a negative impact on its financial condition and its ability to pursue its business strategies as this capital is necessary for the Company to perform the research and development activities required to develop and seek approval for commercialization of the Company’s product candidates, to establish a commercial infrastructure in order to generate future revenue streams, and to successfully commercialize Zilretta. In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued Accounting Standards Update 2015-14, Revenue from Contracts with Customers: Deferral of the Effective Date. This latest standard defers the effective date of revenue standard ASU 2014-09 by one year and permits early adoption on a limited basis. Since the Company has not generated revenue as of September 30, 2017, this guidance will only impact future periods, if any, when revenue is earned. This update will replace existing revenue recognition guidance under GAAP when it becomes effective for the Company beginning January 1, 2018, with early adoption permitted in the first quarter of 2017. The updated standard will permit the use of either the retrospective or cumulative effect transition method. The Company adopted this guidance as of January 1, 2017 and will apply this guidance to any future revenue arrangements. In November 2015, the FASB issued ASU 2015-17, Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases In March 2016, the FASB released ASU 2016-09, which amends ASC Topic 718, Compensation-Stock Compensation, to require changes to several areas of employee share-based payment accounting in an effort to simplify share-based reporting. The update revises requirements in the following areas: minimum statutory withholding, accounting for income taxes, forfeitures, and intrinsic value accounting for private entities. For public companies, the new rules became effective for annual reporting periods beginning after December 15, 2016, and interim reporting periods within such annual period. In August 2016, the FASB issued ASU 2016-15, Statement of cash flows In November, 2016, the FASB issued ASU 2016-18, Statement of cash flows The Company is currently evaluating the impact of this accounting standard on its condensed consolidated financial statements. Consolidation The accompanying condensed consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions for the three and nine months ended September 30, 2017 and the year ended December 31, 2016. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and judgments that may affect the reported amounts of assets and liabilities, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The most significant estimates in these condensed consolidated financial statements include useful lives with respect to long-lived assets, such as property and equipment and leasehold improvements, accounting for stock-based compensation, and accrued expenses, including clinical research costs. The Company’s actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company’s management. Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5 Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and improvements are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Property and equipment includes construction-in-progress that is not yet in service. Foreign Currencies The Company maintains a bank account denominated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s assets that are measured at fair value on a recurring basis as of September 30, 2017 and December 31, 2016 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of September 30, 2017 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 146,175 $ — $ 146,175 Marketable securities — 175,921 — 175,921 $ — $ 322,096 $ — $ 322,096 Fair Value Measurements as of December 31, 2016 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 9,830 $ — $ 9,830 Marketable securities — 179,414 — 179,414 $ — $ 189,244 $ — $ 189,244 As of September 30, 2017 and December 31, 2016 the Company’s cash equivalents that are invested in money market funds and overnight repurchase contracts are valued using Level 2 inputs and primarily rely on quoted prices in active markets for similar securities. The Company measures the fair value of marketable securities, which consist of U.S. government obligations, commercial paper, and corporate bonds, using Level 2 inputs and primarily relies on quoted prices in active markets for similar marketable securities. During the nine months ended September 30, 2017 and year ended December 31, 2016, there were no transfers between Level 1, Level 2, and Level 3. The carrying values of accounts receivable, prepaid expenses, other current assets, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these balances. The Company has a term loan outstanding under its 2015 credit facility with MidCap Financial Funding XIII Trust and Silicon Valley Bank (the “2015 term loan”). The amount outstanding on its 2015 term loan is reported at its carrying value in the accompanying balance sheet. The Company determined the fair value of the 2015 term loan using an income approach that utilizes a discounted cash flow analysis based on current market interest rates for debt issuances with similar remaining years to maturity, adjusted for credit risk. The 2015 term loan was valued using Level 2 inputs as of September 30, 2017 and December 31, 2016. The result of the calculation yielded a fair value that approximates its carrying value. On May 2, 2017 the Company issued 3.375% convertible senior notes due 2024 (the “2024 Convertible Notes”) with embedded conversion features. The Company estimated the fair value of the 2024 Convertible Notes using a discounted cash flow approach to derive the value of a debt instrument using the expected cash flows and the estimated yield related to the convertible notes. The significant assumptions used in estimating the expected cash flows were: the estimated market yield based on an implied yield and credit quality analysis of a term loan with similar attributes, and the average implied volatility of the Company’s traded and quoted options available as of May 2, 2017. The Company recorded approximately $136.7 million as the fair value of the liability on May 2, 2017, with a corresponding amount recorded as a discount on the initial issuance of the 2024 Convertible Notes of approximately $64.5 million. The debt discount was recorded to equity and is being amortized to the debt liability over the life of the 2024 Convertible Notes using the effective interest method. The fair value of the 2024 Convertible Notes, which differs from their carrying value, is influenced by interest rates, stock price and stock price volatility and is determined by prices for the 2024 Convertible Notes observed in market trading. The market for trading of the 2024 Convertible Notes is not considered to be an active market and therefore the estimate of fair value is based on Level 2 inputs. The estimated fair value of the 2024 Convertible Notes, face value of $201.3 million, was $225.4 million at September 30, 2017.

Marketable Securities

Marketable Securities	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities As of September 30, 2017 and December 31, 2016 the fair value of available-for-sale marketable securities by type of security was as follows: September 30, 2017 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. government obligations $ 20,417 $ — $ (3 ) $ 20,414 Commercial paper 17,901 — — 17,901 Corporate bonds 137,667 4 (65 ) 137,606 $ 175,985 $ 4 $ (68 ) $ 175,921 December 31, 2016 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 7,769 $ — $ — $ 7,769 U.S. government obligations 75,524 5 (12 ) 75,517 Corporate bonds 96,193 1 (66 ) 96,128 $ 179,486 $ 6 $ (78 ) $ 179,414 As of September 30, 2017 and December 31, 2016, marketable securities consisted of approximately $175,921,000 and $174,688,000, respectively, of investments that mature within twelve months and as of December 31, 2016 approximately $4,725,000 of investments that mature within fifteen months. As of September 30, 2017 there were no marketable securities with maturities beyond twelve months.

Property and Equipment, Net

Property and Equipment, Net	9 Months Ended
	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	5. Property and Equipment, Net Property and equipment, net, as of September 30, 2017 and December 31, 2016 consisted of the following: (In thousands) September 30, 2017 December 31, 2016 Manufacturing equipment $ 11,520 $ 10,099 Computer and office equipment 858 573 Software 434 434 Construction—in progress 586 1,254 Furniture and fixtures 454 402 Leasehold improvements 461 278 14,313 13,040 Less: Accumulated depreciation (2,832 ) (1,376 ) Total property and equipment, net $ 11,481 $ 11,664 Depreciation expense for the nine months ended September 30, 2017 and 2016 was approximately $1,456,000 and $700,000, respectively. No property and equipment was disposed of during the nine months ended September 30, 2017. Approximately $2,265,000 in manufacturing equipment located at the Evonik facility was disposed of, resulting in a loss of $2,180,000 which was recorded in research and development expenses for the nine months ended September 30, 2016. Construction in progress primarily consists of amounts related to equipment purchased for the Company’s portfolio expansion efforts.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	9 Months Ended
	Sep. 30, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets	6. Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets and other assets consisted of the following as of September 30, 2017 and December 31, 2016: (In thousands) September 30, 2017 December 31, 2016 Prepaid expenses $ 2,907 $ 1,086 Deposits 61 2,099 Interest receivable on marketable securities 641 605 Total prepaid expenses and other current assets $ 3,609 $ 3,790 On December 1, 2016, Flexion paid a refundable NDA fee in the amount of $2,038,100 to the FDA. The Company evaluated each of the published criteria to qualify for a waiver and concluded all criteria were met and thus, obtaining a refund of the fee was probable. As of December 31, 2016 the NDA fee was classified as a deposit in other current assets. On May 16, 2017, Flexion received the full refund of this NDA fee.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	9 Months Ended
	Sep. 30, 2017
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	7. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities consisted of the following: (In thousands) September 30, 2017 December 31, 2016 Research and development $ 1,284 $ 1,606 Payroll and other employee-related expenses 4,946 3,393 Professional services fees 2,444 926 Other 646 159 Interest expense 2,956 161 Total accrued expenses and other current liabilities $ 12,276 $ 6,245

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	8. Stock-Based Compensation Stock Option Valuation The fair value of each of the Company’s stock option grants is estimated on the date of grant using the Black-Scholes option-pricing model. The Company currently estimates its expected stock volatility based on the historical volatility of its publicly-traded peer companies and expects to continue to do so until such time as 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 utilizing 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. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The relevant data used to determine the value of the stock option grants for the nine months ended September 30, 2017 and 2016 are as follows: Nine months ended September 30, 2017 2016 Risk-free interest rates 1.97-2.29% 1.05-1.90% Expected dividend yield 0.00 % 0.00 % Expected term (in years) 6.0 6.0 Expected volatility 69.9-72.8% 79.8-91.1% The following table summarizes stock option activity for the nine months ended September 30, 2017: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2016 3,268 $ 14.84 Granted 982 22.11 Exercised (261 ) 11.47 Cancelled (251 ) 18.48 Outstanding as of September 30, 2017 3,738 $ 17.63 Options vested and expected to vest at September 30, 2017 3,738 $ 17.63 Options exercisable at September 30, 2017 1,450 $ 14.34 Approximately 122,800 outstanding restricted stock units (“RSUs”) are included in stock options outstanding at September 30, 2017. The RSUs are performance based awards which would begin vesting if and when the Company receives approval from the FDA of an NDA for Zilretta (the “Milestone”), which was achieved on October 6, 2017. The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. A total of approximately 261,000 options, with an aggregate intrinsic value of approximately $2,511,800 were exercised during the nine months ended September 30, 2017. At September 30, 2017 and 2016, there were options for the purchase of approximately 3,738,000 and 2,478,000 shares of the Company’s common stock outstanding, respectively, with a weighted average remaining contractual term of 8.0 years and with a weighted average exercise price of $17.63 and $15.11 per share, respectively. The weighted average grant date fair value of options granted during the nine months ended September 30, 2017 and 2016 was $14.19 and $11.90, respectively. Stock-based Compensation The Company recorded stock-based compensation expense related to stock options for the three and nine months ended September 30, 2017 and 2016 as follows: Three months ended September 30, Nine months ended September 30, (In thousands) 2017 2016 2017 2016 Research and development $ 1,057 $ 543 $ 2,859 $ 1,656 General and administrative 1,772 1,160 4,711 3,307 $ 2,829 $ 1,703 $ 7,570 $ 4,963 As of September 30, 2017, unrecognized stock-based compensation expense for stock options outstanding was approximately $26,223,061 which is expected to be recognized over a weighted average period of 2.9 years. Restricted Stock Units On January 4, 2016, the Company granted RSUs with performance and time-based vesting conditions to certain executives. These RSUs vest, and the underlying shares of common stock become deliverable, beginning when Milestone is achieved. As a result of the Milestone being achieved on October 6, 2017, the number of shares of the Company’s common stock earned under these awards is 122,800 with an approximate value of $2,234,960 as of the grant date and will vest over a period of three years. Compensation costs will be recognized over the remaining requisite service period of these awards, beginning on the Milestone achievement date.

Net Loss per Share

Net Loss per Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Net Loss per Share	9. Net Loss per Share Basic and diluted net loss per share was calculated as follows for the three and nine months ended September 30, 2017 and 2016: For the three months ended September 30, For the nine months ended September 30, (In thousands) 2017 2016 2017 2016 Numerator: Net loss $ (34,188 ) $ (17,782 ) $ (86,953 ) $ (48,782 ) Net loss: $ (34,188 ) $ (17,782 ) $ (86,953 ) $ (48,782 ) Denominator: Weighted average common shares outstanding, basic and diluted 31,931 27,524 31,821 23,938 Net loss per share, basic and diluted $ (1.07 ) $ (0.65 ) $ (2.73 ) $ (2.04 ) The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated as including them would have an anti-dilutive effect: For the three months ended September, For the nine months ended September, 2017 2016 2017 2016 Shares issuable upon conversion of the 2024 convertible notes 7,514,937 - 4,171,895 - Stock Options 3,680,096 2,359,370 3,514,511 2,265,023 Restricted stock units 126,219 189,300 168,196 187,220 Total 11,321,252 2,548,670 7,854,602 2,452,243

Debt

Debt	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt	10. Debt Term Loan On August 4, 2015, the Company entered into a credit and security agreement with MidCap Financial Trust, as agent, and MidCap Financial Funding XIII Trust and Silicon Valley Bank, as lenders, (the “Lenders”), to borrow up to $30,000,000 in term loans. The Company concurrently borrowed an initial term loan of $15,000,000 under the facility. The Company granted the Lenders a security interest in substantially all of its personal property, rights and assets, other than intellectual property, to secure the payment of all amounts owed under the credit facility. The Company agreed not to encumber any of its intellectual property without the Lenders’ prior written consent. The Company also agreed to maintain a balance in cash or cash equivalents at Silicon Valley Bank equal to the principal balance of the loan plus 5% for so long as the Company maintains any cash or cash equivalents in non-secured bank accounts. On July 22, 2016, the Company borrowed the remaining $15,000,000 under the credit and security agreement, in the form of a second term loan. The second term loan is subject to the same credit terms as the initial term loan under the facility. The credit and security agreement also contains certain representations, warranties, and covenants of the Company as well as a material adverse event clause. As of September 30, 2017, the Company was compliant with all covenants. Borrowings under the credit facility accrue interest monthly at a fixed interest rate of 6.25% per annum. Following an interest-only period of 19 months, principal will be due in 36 equal monthly installments commencing March 1, 2017 and ending February 1, 2020 (the “maturity date”). Upon the maturity date, the Company will be obligated to pay a final payment equal to 9% of the total principal amounts borrowed under the facility. The final payment amount is being accreted to the carrying value of the debt using the straight line method, which approximates the effective interest method. As of September 30, 2017, the carrying value of the term loan was approximately $25,227,000, of which $9,967,000 was due within 12 months and $15,260,000 was due in greater than 12 months. In connection with the credit and security agreement, the Company incurred debt issuance costs totaling approximately $150,000. These costs are being amortized over the estimated term of the debt using the straight-line method which approximates the effective interest method. The Company deducted the debt issuance costs from the carrying amount of the debt as of September 30, 2017 and December 31, 2016. As of September 30, 2017, annual principal and interest payments due under the 2015 term loan are as follows: Year Aggregate Minimum Payments (in thousands) 2017 $ 2,869 2018 11,082 2019 10,448 2020 4,383 Total $ 28,782 Less interest (855 ) Less final payment (2,700 ) Total $ 25,227 2024 Convertible Notes On May 2, 2017 the Company issued an aggregate of $201.3 million principal amount of the 2024 Convertible Notes. The 2024 Convertible Notes have a maturity date of May 1, 2024 are unsecured and accrue interest at a rate of 3.375% per annum, payable semi-annually on May 1 and November 1 of each year, beginning November 1, 2017. The Company received $194.8 million for the sale of the 2024 Convertible Notes, after deducting fees and expenses of $6.5 million. The 2024 Convertible Notes are senior unsecured obligations and bear interest at a rate of 3.375% per year, payable semi-annually in arrears on May and November 1st of each year. The 2024 Convertible Notes will mature on May 1, 2024, unless earlier repurchased or converted. Upon conversion of the 2024 Convertible Notes, at the election of each holder of a 2024 Convertible Note (the Holder), the note will be convertible into cash, shares of the Company’s common stock, or a combination thereof, at the Company’s election (subject to certain limitations in the 2015 term loan), at a conversion rate of approximately 37.3413 shares of common stock per $1,000 principal amount of the 2024 Convertible Notes, which corresponds to an initial conversion price of approximately $26.78 per share of the Company’s common stock. The Conversion Rate is subject to adjustment from time to time upon the occurrence of certain events, including, but not limited to, fundamental change events and certain corporate events that occur prior to the maturity date of the notes. In addition, if the Company delivers a notice of redemption, the Company will increase, in certain circumstances, the conversion rate for a Holder who elects to convert its notes in connection with such a corporate event or notice of redemption, as the case may be. At any time prior to the close of business on the business day immediately preceding February 1, 2024, Holders may convert all, or any portion, of the 2024 Convertible Notes at their option only under the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on June 30, 2017 (and only during such calendar quarter), if the last reported sale price of the common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the “measurement period”) in which the trading price per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate on each such trading day; (3) if the Company calls any or all of the notes for redemption, at any time prior to the close of business on the business day immediately preceding the redemption date; and (4) upon the occurrence of specified corporate events. On or after February 1, 2024, until the close of business on the business day immediately preceding the maturity date, holders may convert their notes at any time, regardless of the foregoing circumstances. The Company may redeem, for cash, all or any portion of the 2024 Convertible Notes, at its option, on or after May 6, 2020 if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price for at least 20 trading days during any 30 consecutive day trading period, at a redemption price equal to 100% of the principal amount of the 2024 Convertible Notes to be redeemed, plus accrued and unpaid interest. The 2024 Convertible Notes are considered convertible debt with a cash conversion feature. Per ASC 470-20, Debt with Conversion and Other Options , The carrying amount of the liability component was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The allocation was performed in a manner that reflected our non-convertible debt borrowing rate for similar debt. The equity component of the 2024 Convertible Notes was recognized as a debt discount and represents the difference between the proceeds from the issuance of the 2024 Convertible Notes and the fair value of the liability of the 2024 Convertible Notes on their respective dates of issuance. The excess of the principal amount of the liability component over its carrying amount (“debt discount”) is amortized to interest expense using the effective interest method over seven years. The equity component is not re-measured as long as it continues to meet the conditions for equity classification. In connection with the issuance of the 2024 Convertible Notes, the Company incurred approximately $6.5 million of debt issuance costs, which primarily consisted of underwriting, legal and other professional fees, and allocated these costs to the liability and equity components based on the allocation of the proceeds. Of the total d ebt issuance costs, $4.4 million were allocated to the liability component and are recorded as a reduction of the 2024 Convertible Notes in our consolidated balance sheets. The remaining $2.1 million was allocated to the equity component and is recorded as a reduction to additional paid-in capital. Debt discount and issuance rate The table below summarizes the carrying value of the 2024 Convertible Notes as of September 30, 2017: ( in thousands Gross proceeds $ 201,250 Portion allocated to equity (additional paid-in capital) (64,541 ) Debt issuance costs (6,470 ) Portion allocated to equity (additional paid-in capital) 2,075 Amortization of debt discount and debt issuance costs 2,961 Carrying value 2024 Convertible Notes $ 135,275

Foreign Currency

Foreign Currency	9 Months Ended
	Sep. 30, 2017
Foreign Currency [Abstract]
Foreign Currency	11. Foreign Currency The Company maintains a bank account denominated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations. Foreign currency losses for the three and nine months ended September 30, 2017 were $0.1 million and $0.5 million, respectively, compared to $0.6 million for the three and nine months ended September 30, 2016.

Subsequent Event

Subsequent Event	9 Months Ended
	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Event	12. Subsequent Event Financing On October 16, 2017, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,520,000 shares of the Company’s common stock at a price to the public of $25.50 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase 720,000 additional shares. The Company received net proceeds from the follow-on financing of $132.4 million after deducting underwriting discounts, commissions, and offering costs paid by the Company.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	13. Commitments and Contingencies Operating Leases Burlington Lease In May 2013, the Company entered into a lease for office space in Burlington, Massachusetts (the “Lease”). The term of the Lease was for 42 months with minimum monthly lease payments beginning at $17,588 per month and escalating over the term of the Lease. In July 2015, the Company amended the Lease to add approximately 4,700 square feet of additional office space, with the option to lease an additional 5,400 square feet in the same building in Burlington, Massachusetts (the “Amendment”). In addition, at the time, The Company leased approximately 6,700 square feet of temporary space for use prior to delivery of the additional space. The Amendment also extended the term of the Lease through October 31, 2019. In addition, at the same time, The Company leased approximately 6,700 square feet of temporary space for use prior to delivery of the additional space under the Amendment. On September 30, 2015, the Company exercised its option for the additional 5,400 square feet of office space under the Amendment. On September 21, 2016, the Company entered into another amendment to extend the Lease for the 6,700 square feet of temporary space until October 31, 2017. On April 7, 2017, the Company further amended the Lease to extend the term to October 31, 2023 on the then-existing office space, including the temporary space, consisting of approximately 28,600 square feet of office space in Burlington, Massachusetts. From November 2016 through October 2017, the Company’s lease payment for this space was approximately $80,000 per month. Also, as part of this amendment to the Lease, the Company leased an additional 1,471 square feet of office space beginning in 2018. The lease payment for the 1,471 square feet of office space is approximately $4,100 per month. On October 6, 2017, the Company exercised its option for an additional 6,450 square feet of space, with the term expected to commence on or about April 1, 2018. After April 2018, the Company will have approximately 36,500 square feet of office space in Burlington, Massachusetts under a lease term expiring on October 31, 2023. In addition to the base rent for the office space, which increases over the term of the amended Lease, the Company is responsible for its share of operating expenses and real estate taxes. Woburn Lease In February 2017, the Company entered into a five-year lease for laboratory space located in Woburn, Massachusetts with a monthly lease payment of approximately $15,000, which increases over the term of the lease, plus a share of operating expenses. The total cash obligations for the term of the lease are approximately $0.9 million. Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments 2017 $ 293,440 2018 1,341,672 2019 1,491,101 2020 1,533,276 2021 1,575,620 2022 1,446,770 2023 1,202,874 Total $ 8,884,753 1

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying condensed consolidated financial statements as of September 30, 2017, and for the three and nine months ended September 30, 2017 and 2016, have been prepared in accordance with the rules and regulations of the Securities and Exchange Commission (the “SEC”) and Generally Accepted Accounting Principles (“GAAP”) for consolidated financial information including the accounts of the Company and its wholly-owned subsidiary after elimination of all significant intercompany accounts and transactions. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of management, these condensed consolidated financial statements reflect all adjustments which are necessary for a fair statement of the Company’s financial position and results of its operations, as of and for the periods presented. These condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K filed with the SEC on March 10, 2017. The information presented in the condensed consolidated financial statements and related notes as of September 30, 2017, and for the three and nine months ended September 30, 2017 and 2016, is unaudited. The December 31, 2016 consolidated balance sheet included herein was derived from the audited financial statements as of that date, but does not include all disclosures, including notes, required by GAAP for complete financial statements. Interim results for the three and nine months ended September 30, 2017 are not necessarily indicative of the results that may be expected for the fiscal year ending December 31, 2017, or any future period. The accompanying condensed consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. The Company has incurred recurring losses and negative cash flows from operations. As of September 30, 2017, the Company had cash, cash equivalents, marketable securities, and long-term investments of approximately $335,100,000. Management believes that current cash, cash equivalents and marketable securities on hand at September 30, 2017, together with the net proceeds of its October 2017 common stock offering of approximately $132,400,000 described in note 12, should be sufficient to fund operations for at least the next twelve months from the issuance date of these financial statements. The future viability of the Company is dependent on sales revenue from Zilretta and its ability to raise additional capital to finance its operations, to fund increased research and development costs in order to seek approval for commercialization of its product candidates, and to successfully commercialize Zilretta. The Company’s failure to raise capital as and when needed would have a negative impact on its financial condition and its ability to pursue its business strategies as this capital is necessary for the Company to perform the research and development activities required to develop and seek approval for commercialization of the Company’s product candidates, to establish a commercial infrastructure in order to generate future revenue streams, and to successfully commercialize Zilretta. In May 2014, the FASB issued guidance which supersedes all existing revenue recognition requirements, including most industry-specific guidance. The new standard requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued Accounting Standards Update 2015-14, Revenue from Contracts with Customers: Deferral of the Effective Date. This latest standard defers the effective date of revenue standard ASU 2014-09 by one year and permits early adoption on a limited basis. Since the Company has not generated revenue as of September 30, 2017, this guidance will only impact future periods, if any, when revenue is earned. This update will replace existing revenue recognition guidance under GAAP when it becomes effective for the Company beginning January 1, 2018, with early adoption permitted in the first quarter of 2017. The updated standard will permit the use of either the retrospective or cumulative effect transition method. The Company adopted this guidance as of January 1, 2017 and will apply this guidance to any future revenue arrangements. In November 2015, the FASB issued ASU 2015-17, Income Taxes In February 2016, the FASB issued ASU 2016-02, Leases In March 2016, the FASB released ASU 2016-09, which amends ASC Topic 718, Compensation-Stock Compensation, to require changes to several areas of employee share-based payment accounting in an effort to simplify share-based reporting. The update revises requirements in the following areas: minimum statutory withholding, accounting for income taxes, forfeitures, and intrinsic value accounting for private entities. For public companies, the new rules became effective for annual reporting periods beginning after December 15, 2016, and interim reporting periods within such annual period. In August 2016, the FASB issued ASU 2016-15, Statement of cash flows In November, 2016, the FASB issued ASU 2016-18, Statement of cash flows The Company is currently evaluating the impact of this accounting standard on its condensed consolidated financial statements.
Consolidation	Consolidation The accompanying condensed consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions for the three and nine months ended September 30, 2017 and the year ended December 31, 2016.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and judgments that may affect the reported amounts of assets and liabilities, expenses and related disclosures. The Company bases estimates and judgments on historical experience and on various other factors that it believes to be reasonable under the circumstances. The most significant estimates in these condensed consolidated financial statements include useful lives with respect to long-lived assets, such as property and equipment and leasehold improvements, accounting for stock-based compensation, and accrued expenses, including clinical research costs. The Company’s actual results may differ from these estimates under different assumptions or conditions. The Company evaluates its estimates on an ongoing basis. Changes in estimates are reflected in reported results in the period in which they become known by the Company’s management.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5 Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the related asset. Costs of major additions and improvements are capitalized and depreciated on a straight-line basis over their useful lives. Repairs and maintenance costs are expensed as incurred. Upon retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is credited or charged to income. Property and equipment includes construction-in-progress that is not yet in service.
Foreign Currencies	Foreign Currencies The Company maintains a bank account denominated in British Pounds. All foreign currency payables and cash balances are measured at the applicable exchange rate at the end of the reporting period. All associated gains and losses from foreign currency transactions are reflected in the consolidated statements of operations.

Summary of Significant Accoun22

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Property Plant and Equipment Estimated Useful Lives	Property and equipment are stated at cost less accumulated depreciation. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Estimated Useful Life (Years) Computers, office equipment, and minor computer software 3 Computer software 7 Manufacturing equipment 7-10 Furniture and fixtures 5

Fair Value of Financial Asset23

Fair Value of Financial Assets and Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Schedule of Assets Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets that are measured at fair value on a recurring basis as of September 30, 2017 and December 31, 2016 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of September 30, 2017 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 146,175 $ — $ 146,175 Marketable securities — 175,921 — 175,921 $ — $ 322,096 $ — $ 322,096 Fair Value Measurements as of December 31, 2016 Using: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 9,830 $ — $ 9,830 Marketable securities — 179,414 — 179,414 $ — $ 189,244 $ — $ 189,244

Marketable Securities (Tables)

Marketable Securities (Tables)	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of September 30, 2017 and December 31, 2016 the fair value of available-for-sale marketable securities by type of security was as follows: September 30, 2017 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. government obligations $ 20,417 $ — $ (3 ) $ 20,414 Commercial paper 17,901 — — 17,901 Corporate bonds 137,667 4 (65 ) 137,606 $ 175,985 $ 4 $ (68 ) $ 175,921 December 31, 2016 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial paper $ 7,769 $ — $ — $ 7,769 U.S. government obligations 75,524 5 (12 ) 75,517 Corporate bonds 96,193 1 (66 ) 96,128 $ 179,486 $ 6 $ (78 ) $ 179,414

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	9 Months Ended
	Sep. 30, 2017
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment, net, as of September 30, 2017 and December 31, 2016 consisted of the following: (In thousands) September 30, 2017 December 31, 2016 Manufacturing equipment $ 11,520 $ 10,099 Computer and office equipment 858 573 Software 434 434 Construction—in progress 586 1,254 Furniture and fixtures 454 402 Leasehold improvements 461 278 14,313 13,040 Less: Accumulated depreciation (2,832 ) (1,376 ) Total property and equipment, net $ 11,481 $ 11,664

Prepaid Expenses and Other Cu26

Prepaid Expenses and Other Current Assets (Tables)	9 Months Ended
	Sep. 30, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses and Other Current Assets and Other Assets	Prepaid expenses and other current assets and other assets consisted of the following as of September 30, 2017 and December 31, 2016: (In thousands) September 30, 2017 December 31, 2016 Prepaid expenses $ 2,907 $ 1,086 Deposits 61 2,099 Interest receivable on marketable securities 641 605 Total prepaid expenses and other current assets $ 3,609 $ 3,790

Accrued Expenses and Other Cu27

Accrued Expenses and Other Current Liabilities (Tables)	9 Months Ended
	Sep. 30, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities consisted of the following: (In thousands) September 30, 2017 December 31, 2016 Research and development $ 1,284 $ 1,606 Payroll and other employee-related expenses 4,946 3,393 Professional services fees 2,444 926 Other 646 159 Interest expense 2,956 161 Total accrued expenses and other current liabilities $ 12,276 $ 6,245

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants	The relevant data used to determine the value of the stock option grants for the nine months ended September 30, 2017 and 2016 are as follows: Nine months ended September 30, 2017 2016 Risk-free interest rates 1.97-2.29% 1.05-1.90% Expected dividend yield 0.00 % 0.00 % Expected term (in years) 6.0 6.0 Expected volatility 69.9-72.8% 79.8-91.1%
Summary of Stock Option Activity	The following table summarizes stock option activity for the nine months ended September 30, 2017: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2016 3,268 $ 14.84 Granted 982 22.11 Exercised (261 ) 11.47 Cancelled (251 ) 18.48 Outstanding as of September 30, 2017 3,738 $ 17.63 Options vested and expected to vest at September 30, 2017 3,738 $ 17.63 Options exercisable at September 30, 2017 1,450 $ 14.34
Stock-Based Compensation Expense Related to Stock Options	The Company recorded stock-based compensation expense related to stock options for the three and nine months ended September 30, 2017 and 2016 as follows: Three months ended September 30, Nine months ended September 30, (In thousands) 2017 2016 2017 2016 Research and development $ 1,057 $ 543 $ 2,859 $ 1,656 General and administrative 1,772 1,160 4,711 3,307 $ 2,829 $ 1,703 $ 7,570 $ 4,963

Net Loss per Share (Tables)

Net Loss per Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss per Share	Basic and diluted net loss per share was calculated as follows for the three and nine months ended September 30, 2017 and 2016: For the three months ended September 30, For the nine months ended September 30, (In thousands) 2017 2016 2017 2016 Numerator: Net loss $ (34,188 ) $ (17,782 ) $ (86,953 ) $ (48,782 ) Net loss: $ (34,188 ) $ (17,782 ) $ (86,953 ) $ (48,782 ) Denominator: Weighted average common shares outstanding, basic and diluted 31,931 27,524 31,821 23,938 Net loss per share, basic and diluted $ (1.07 ) $ (0.65 ) $ (2.73 ) $ (2.04 )
Schedule of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share, as Including them Would have Anti-dilutive Effect	The following common stock equivalents were excluded from the calculation of diluted net loss per share for the periods indicated as including them would have an anti-dilutive effect: For the three months ended September, For the nine months ended September, 2017 2016 2017 2016 Shares issuable upon conversion of the 2024 convertible notes 7,514,937 - 4,171,895 - Stock Options 3,680,096 2,359,370 3,514,511 2,265,023 Restricted stock units 126,219 189,300 168,196 187,220 Total 11,321,252 2,548,670 7,854,602 2,452,243

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Schedule of Annual Principal and Interest Payments Due Under Term Loan	As of September 30, 2017, annual principal and interest payments due under the 2015 term loan are as follows: Year Aggregate Minimum Payments (in thousands) 2017 $ 2,869 2018 11,082 2019 10,448 2020 4,383 Total $ 28,782 Less interest (855 ) Less final payment (2,700 ) Total $ 25,227
Summary of Carrying Value of Convertible Notes	The table below summarizes the carrying value of the 2024 Convertible Notes as of September 30, 2017: ( in thousands Gross proceeds $ 201,250 Portion allocated to equity (additional paid-in capital) (64,541 ) Debt issuance costs (6,470 ) Portion allocated to equity (additional paid-in capital) 2,075 Amortization of debt discount and debt issuance costs 2,961 Carrying value 2024 Convertible Notes $ 135,275

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2017
Commitments And Contingencies Disclosure [Abstract]
Future Minimum Lease Payments under Operating Leases	Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments 2017 $ 293,440 2018 1,341,672 2019 1,491,101 2020 1,533,276 2021 1,575,620 2022 1,446,770 2023 1,202,874 Total $ 8,884,753

Overview and Nature of the Bu32

Overview and Nature of the Business - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Incorporation date	Nov. 5, 2007

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Oct. 16, 2017	Sep. 30, 2016	Sep. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents and marketable securities			$ 335,100,000
Net proceeds from offering of common stock		$ 77,644,000
Follow On Public Offering [Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Net proceeds from offering of common stock	$ 132,400,000

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	9 Months Ended
	Sep. 30, 2017
Computers, Office Equipment, and Minor Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	3 years
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	7 years
Manufacturing Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	7 years
Manufacturing Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	10 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life (Years)	5 years

Fair Value of Financial Asset35

Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 146,175	$ 9,830
Marketable securities	175,921	179,414
Assets, Total	322,096	189,244
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	146,175	9,830
Marketable securities	175,921	179,414
Assets, Total	$ 322,096	$ 189,244

Fair Value of Financial Asset36

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	May 02, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0
2024 Convertible Notes [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of liability			$ 136,700,000
Unamortized debt discount			$ 64,500,000
Debt instrument interest rate	3.375%		3.375%
Convertible Notes, face value			$ 201,300,000
Estimated fair value of Convertible Notes	$ 225,400,000

Marketable Securities - Summary

Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 175,985	$ 179,486
Gross Unrealized Gains	4	6
Gross Unrealized Losses	(68)	(78)
Fair Value	175,921	179,414
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	20,417	75,524
Gross Unrealized Gains		5
Gross Unrealized Losses	(3)	(12)
Fair Value	20,414	75,517
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,901	7,769
Fair Value	17,901	7,769
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	137,667	96,193
Gross Unrealized Gains	4	1
Gross Unrealized Losses	(65)	(66)
Fair Value	$ 137,606	$ 96,128

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 175,921,000	$ 174,688,000
Marketable securities, noncurrent	$ 0	$ 4,725,000

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 14,313	$ 13,040
Less: Accumulated depreciation	(2,832)	(1,376)
Total property and equipment, net	11,481	11,664
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	11,520	10,099
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	858	573
Construction-in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	586	1,254
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	454	402
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	461	278
Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 434	$ 434

Property and Equipment, Net - A

Property and Equipment, Net - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Property, Plant and Equipment [Line Items]
Depreciation	$ 1,456,000	$ 700,000
Property and equipment disposals	$ 0
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Gain (Loss) on sale of property and equipment		(2,180,000)
Manufacturing Equipment [Member] | Evonik Corporation [Member]
Property, Plant and Equipment [Line Items]
Property and equipment disposals		$ 2,265,000

Prepaid Expenses and Other Cu41

Prepaid Expenses and Other Current Assets - Prepaid Expenses and Other Current Assets and Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 2,907	$ 1,086
Deposits	61	2,099
Interest receivable on marketable securities	641	605
Total prepaid expenses and other current assets	$ 3,609	$ 3,790

Prepaid Expenses and Other Cu42

Prepaid Expenses and Other Current Assets - Additional Information (Detail)	Dec. 01, 2016USD ($)
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Refundable NDA fee paid to FDA	$ 2,038,100

Accrued Expenses and Other Cu43

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Research and development	$ 1,284	$ 1,606
Payroll and other employee-related expenses	4,946	3,393
Professional services fees	2,444	926
Other	646	159
Interest expense	2,956	161
Total accrued expenses and other current liabilities	$ 12,276	$ 6,245

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rates, Minimum	1.97%	1.05%
Risk-free interest rates, Maximum	2.29%	1.90%
Expected dividend yield	0.00%	0.00%
Expected term (in years)	6 years	6 years
Expected volatility, Minimum	69.90%	79.80%
Expected volatility, Maximum	72.80%	91.10%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail)	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Issuable Under Options, Beginning balance | shares	3,268,000
Shares Issuable Under Options, Granted | shares	982,000
Shares Issuable Under Options, Exercised | shares	(261,000)
Shares Issuable Under Options, Cancelled | shares	(251,000)
Shares Issuable Under Options, Ending balance | shares	3,738,000
Shares Issuable Under Options, Options vested and expected to vest at June 30, 2017 | shares	3,738,000
Shares Issuable Under Options, Options exercisable at September 30, 2017 | shares	1,450,000
Weighted Average Exercise Price, Beginning balance | $ / shares	$ 14.84
Weighted Average Exercise Price, Granted | $ / shares	22.11
Weighted Average Exercise Price, Exercised | $ / shares	11.47
Weighted Average Exercise Price, Cancelled | $ / shares	18.48
Weighted Average Exercise Price, Ending balance | $ / shares	17.63
Weighted Average Exercise Price, Options vested and expected to vest | $ / shares	17.63
Weighted Average Exercise Price, Options exercisable | $ / shares	$ 14.34

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Oct. 06, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options exercised		261,000
Aggregate intrinsic value of stock options exercised		$ 2,511,800
Options to purchase common stock, outstanding		3,738,000	2,478,000	3,268,000
Weighted average remaining contractual term		8 years	8 years
Weighted average exercise price		$ 17.63	$ 15.11	$ 14.84
Weighted average grant date fair value of options granted		$ 14.19	$ 11.90
Unrecognized stock-based compensation expense		$ 26,223,061
Unrecognized compensation costs, weighted-average recognition periods		2 years 10 months 25 days
Performance-Based Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate number of outstanding restricted common stock units		122,800
Restricted Stock Units (RSUs) [Member] | Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of restricted common stock units granted under performance-based vesting	122,800
Grant date fair value of restricted common stock units under performance-based vesting	$ 2,234,960
Vesting period	3 years

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,829	$ 1,703	$ 7,570	$ 4,963
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,057	543	2,859	1,656
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,772	$ 1,160	$ 4,711	$ 3,307

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (34,188)	$ (17,782)	$ (86,953)	$ (48,782)	$ (71,894)	$ (46,315)
Net loss:	$ (34,188)	$ (17,782)	$ (86,953)	$ (48,782)	$ (71,894)	$ (46,315)
Denominator:
Weighted average common shares outstanding, basic and diluted	31,931	27,524	31,821	23,938
Net loss per share, basic and diluted	$ (1.07)	$ (0.65)	$ (2.73)	$ (2.04)

Net Loss per Share - Schedule49

Net Loss per Share - Schedule of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share, as Including them Would have Anti-dilutive Effect (Detail) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	11,321,252	2,548,670	7,854,602	2,452,243
Shares Issuable Upon Conversion of the 2024 Convertible Notes [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	7,514,937		4,171,895
Stock Options [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	3,680,096	2,359,370	3,514,511	2,265,023
Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share, as including them would have anti-dilutive effect	126,219	189,300	168,196	187,220

Debt - Additional Information (

Debt - Additional Information (Detail)	May 02, 2017USD ($)	Jul. 22, 2016USD ($)	Aug. 04, 2015USD ($)	Sep. 30, 2017USD ($)$ / shares	Sep. 30, 2017USD ($)d$ / sharesshares	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Term loan due within 12 months				$ 9,967,000	$ 9,967,000		$ 9,134,000
Term loan due in greater than 12 months				15,260,000	15,260,000		$ 21,399,000
Equity component of Convertible Notes recorded as additional paid-in capital					62,466,000
Debt issuance costs					6,470,000	$ 42,000
Amortization of premium (discount) on marketable securities					355,000	$ 544,000
2015 Term Loan [Member]
Debt Instrument [Line Items]
Carrying value of term loan				25,227,000	25,227,000
Liability component of Convertible Notes recorded as long-term debt				28,782,000	$ 28,782,000
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member]
Debt Instrument [Line Items]
Term loan, maximum borrowings			$ 30,000,000
Debt instrument description					The Company also agreed to maintain a balance in cash or cash equivalents at Silicon Valley Bank equal to the principal balance of the loan plus 5%.
Debt instrument percentage of cash or cash equivalents addition to principal balance			5.00%
Debt instrument interest rate			6.25%
Term loan, first periodic payment date			Mar. 1, 2017
Debt instrument maturity date			Feb. 1, 2020
Interest on final payment			9.00%
Term loan, payment description					Interest-only period of 19 months, principal will be due in 36 equal monthly installments.
Carrying value of term loan				25,227,000	$ 25,227,000
Term loan due within 12 months				9,967,000	9,967,000
Term loan due in greater than 12 months				$ 15,260,000	$ 15,260,000
Debt issuance costs			$ 150,000
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Interest-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period			19 months
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Principal-Only-Strip [Member]
Debt Instrument [Line Items]
Term loan, Interest payment period			36 months
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Initial Term Loan [Member]
Debt Instrument [Line Items]
Term loan, currently borrowed			$ 15,000,000
Mid Cap Financial Trust [Member] | 2015 Term Loan [Member] | Second Term Loan [Member]
Debt Instrument [Line Items]
Term loan remaining amount borrowed		$ 15,000,000
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.375%			3.375%	3.375%
Debt instrument maturity date	May 1, 2024
Interest on final payment	3.375%			9.71%	9.71%
Debt issuance costs	$ 6,500,000			$ 68,900,000	$ 68,900,000
Debt instrument principal amount	201,300,000
Debt instrument frequency of periodic payment					semi-annually
Net proceeds from offering of convertible senior notes	194,800,000
Debt conversion, converted instrument, shares issued | shares					37.3413
Debt instrument principal amount denomination for conversion in to common stock | $ / shares				$ 1,000	$ 1,000
Debt instrument convertible initial conversion price per share of common stock | $ / shares				$ 26.78	$ 26.78
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible into common stock | d					20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible into common stock | d					30
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible					130.00%
Number of consecutive business days immediately after any five consecutive trading day period during the note measurement period | d					5
Number of consecutive trading days before five consecutive business days during the note measurement period | d					10
Liability component of Convertible Notes recorded as long-term debt	136,700,000
Equity component of Convertible Notes recorded as additional paid-in capital	$ 64,500,000
Debt instrument effective interest rate period					7 years
Debt issuance costs					$ 6,500,000
Debt issuance costs allocated to liability component					4,400,000
Debt issuance costs allocated to equity component reduction to additional paid-in capital					$ 2,100,000
Interest expense				$ 3,488,296
Amortization of premium (discount) on marketable securities				$ 1,658,620
2024 Convertible Notes [Member] | On or After February 1, 2024 [Member]
Debt Instrument [Line Items]
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be convertible into common stock | d					20
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be convertible into common stock | d					30
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order for the notes to be convertible					130.00%
Debt instrument redemption date					May 6, 2020
Redemption price percentage of principal amount to be redeemed					100.00%
2024 Convertible Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument conversion obligation trading price as percentage of product common stock closing sale price and conversion rate					98.00%

Debt - Schedule of Annual Princ

Debt - Schedule of Annual Principal and Interest Payments Due Under Term Loan (Detail) - 2015 Term Loan [Member] $ in Thousands	Sep. 30, 2017USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2,017	$ 2,869
2,018	11,082
2,019	10,448
2,020	4,383
Total	28,782
Less interest	(855)
Less final payment	(2,700)
Total	$ 25,227

Debt - Summary of Carrying Valu

Debt - Summary of Carrying Value of Convertible Notes (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Debt Instrument [Line Items]
Gross proceeds	$ 201,250
Amortization of debt discount and debt issuance costs	2,985	$ 26
Carrying value 2024 Convertible Notes	135,275
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Gross proceeds	201,250
Gross Proceeds, 'Portion allocated to equity (additional paid-in capital)	(64,541)
Debt issuance costs	(6,470)
Debt issuance costs, 'Portion allocated to equity (additional paid-in capital)	2,075
Amortization of debt discount and debt issuance costs	2,961
Carrying value 2024 Convertible Notes	$ 135,275

Foreign Currency - Additional I

Foreign Currency - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Foreign Currency [Abstract]
Foreign currency losses	$ 0.1	$ 0.6	$ 0.5	$ 0.6

Subsequent Event - Additional I

Subsequent Event - Additional Information (Detail) - USD ($)	Oct. 16, 2017	Sep. 30, 2016
Subsequent Event [Line Items]
Net proceeds from follow-on public offering		$ 77,644,000
Subsequent Event [Member] | Follow On Public Offering [Member]
Subsequent Event [Line Items]
Common stock, shares issued	5,520,000
Sale of common stock price per share	$ 25.50
Net proceeds from follow-on public offering	$ 132,400,000
Subsequent Event [Member] | Underwriters' Option in Follow-on Public Offering [Member]
Subsequent Event [Line Items]
Common stock, shares issued	720,000

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	May 01, 2018ft²	Oct. 06, 2017ft²	Apr. 07, 2017USD ($)ft²	Feb. 28, 2017USD ($)	Sep. 21, 2016ft²	Jul. 13, 2015ft²	May 31, 2013USD ($)	Oct. 31, 2017USD ($)	Sep. 30, 2015ft²
Commitment And Contingencies [Line Items]
Operating lease term							42 months
Minimum monthly lease payments | $							$ 17,588
Burlington Massachusetts [Member]
Commitment And Contingencies [Line Items]
Additional leased office space			1,471			4,700
Extended leased office space						5,400			5,400
Office space leased on temporary agreement					6,700	6,700
Lease termination date			Oct. 31, 2023			Oct. 31, 2019
Temporary lease termination date					Oct. 31, 2017
Office space leased			28,600
Lease payment per month for additional office space | $			$ 4,100
Burlington Massachusetts [Member] | Scenario, Forecast [Member]
Commitment And Contingencies [Line Items]
Lease termination date	Oct. 31, 2023
Operating lease office space	36,500
Burlington Massachusetts [Member] | Subsequent Event [Member]
Commitment And Contingencies [Line Items]
Additional leased office space		6,450
Lease payment per month for office space | $								$ 80,000
Operating lease expected commencement date		Apr. 1, 2018
Woburn, Massachusetts [Member]
Commitment And Contingencies [Line Items]
Operating lease term				5 years
Monthly lease payment | $				$ 15,000
Total cash obligations for lease | $				$ 900,000

Commitments and Contingencies56

Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail)	Sep. 30, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,017	$ 293,440
2,018	1,341,672
2,019	1,491,101
2,020	1,533,276
2,021	1,575,620
2,022	1,446,770
2,023	1,202,874
Total	$ 8,884,753