Flexion Therapeutics (FLXN) 10-K 8 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Mar. 01, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	FLXN
Entity Registrant Name	Flexion Therapeutics Inc
Entity Central Index Key	1,419,600
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		37,619,452
Entity Public Float			$ 557,701,832

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 127,789	$ 30,915
Marketable securities	264,589	174,688
Accounts receivable, net	410
Inventories	1,799
Prepaid expenses and other current assets	3,403	3,790
Total current assets	397,990	209,393
Property and equipment, net	11,189	11,664
Long-term investments	31,538	4,725
Restricted cash	600	480
Total assets	441,317	226,262
Current liabilities:
Accounts payable	6,222	2,161
Accrued expenses and other current liabilities	14,383	6,245
Current portion of long-term debt	9,967	9,134
Total current liabilities	30,572	17,540
Long-term debt, net	12,936	21,399
2024 convertible notes, net	137,107
Other long-term liabilities	428	291
Total liabilities	181,043	39,230
Commitments and contingencies
Preferred Stock, $0.001 par value; 10,000,000 shares authorized at December 31, 2017 and December 31, 2016 and 0 shares issued and outstanding at December 31, 2017 and December 31, 2016	0	0
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized; 37,610,897 and 31,667,469 shares issued and outstanding, at December 31, 2017 and December 31, 2016, respectively	38	32
Additional paid-in capital	609,810	398,757
Accumulated other comprehensive loss	(407)	(71)
Accumulated deficit	(349,167)	(211,686)
Total stockholders’ equity	260,274	187,032
Total liabilities and stockholders’ equity	$ 441,317	$ 226,262

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 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	37,610,897	31,667,469
Common stock, shares outstanding	37,610,897	31,667,469

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Product revenue, net	$ 355
Operating expenses:
Cost of sales	4
Research and development	51,231	$ 41,314	$ 32,691
Selling, general and administrative	78,801	28,466	13,372
Total operating expenses	130,036	69,780	46,063
Loss from operations	(129,681)	(69,780)	(46,063)
Other income (expense):
Interest income	3,718	1,521	1,246
Interest expense	(11,268)	(1,748)	(571)
Other expense	(250)	(1,887)	(927)
Total other income (expense)	(7,800)	(2,114)	(252)
Net loss	$ (137,481)	$ (71,894)	$ (46,315)
Net loss per common share, basic and diluted	$ (4.16)	$ (2.84)	$ (2.15)
Weighted average common shares outstanding, basic and diluted	33,027	25,297	21,497
Other comprehensive income (loss) :
Unrealized gains (losses) from available-for-sale securities, net of tax of $0	$ (336)	$ 26	$ (92)
Total other comprehensive income (loss)	(336)	26	(92)
Comprehensive loss	$ (137,817)	$ (71,868)	$ (46,407)

Consolidated Statements of Ope5

Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Unrealized gains (losses) from available-for-sale securities, tax	$ 0	$ 0	$ 0

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ 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		21,440,000
Issuance of common stock for equity awards	$ 593	$ 1	592
Issuance of common stock for equity awards (in shares)	109,441	109,000
Employee Stock Purchase Plan	$ 276		276
Employee Stock Purchase Plan (in shares)		21,000
Stock-based compensation expense	4,583		4,583
Net loss	(46,315)				(46,315)
Other comprehensive income (loss)	(92)			(92)
Balance at Dec. 31, 2015	103,986	$ 22	243,853	(97)	(139,792)
Balance (in shares) at Dec. 31, 2015		21,570,000
Issuance of Common Stock net of issuance costs	147,501	$ 10	147,491
Issuance of Common Stock net of issuance costs (in shares)		10,040,000
Issuance of common stock for equity awards	$ 167		167
Issuance of common stock for equity awards (in shares)	30,195	30,000
Employee Stock Purchase Plan	$ 476		476
Employee Stock Purchase Plan (in shares)		27,000
Stock-based compensation expense	6,770		6,770
Net loss	(71,894)				(71,894)
Other comprehensive income (loss)	26			26
Balance at Dec. 31, 2016	187,032	$ 32	398,757	(71)	(211,686)
Balance (in shares) at Dec. 31, 2016		31,667,000
Issuance of Common Stock net of issuance costs	132,177	$ 6	132,171
Issuance of Common Stock net of issuance costs (in shares)		5,520,000
Issuance of common stock for equity awards	$ 3,858		3,858
Issuance of common stock for equity awards (in shares)	308,011	334,000
Employee Stock Purchase Plan	$ 1,016		1,016
Employee Stock Purchase Plan (in shares)		90,000
Stock-based compensation expense	11,542		11,542
Portion of convertible debt proceeds allocated to equity component	62,466		62,466
Net loss	(137,481)				(137,481)
Other comprehensive income (loss)	(336)			(336)
Balance at Dec. 31, 2017	$ 260,274	$ 38	$ 609,810	$ (407)	$ (349,167)
Balance (in shares) at Dec. 31, 2017		37,611,000

Consolidated Statements of Cash

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	$ (137,481)	$ (71,894)	$ (46,315)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation	2,008	1,151	238
Stock-based compensation expense	11,542	6,770	4,583
Other non-cash charges		37	40
Amortization of premium (discount) on marketable	333	729	871
Loss on disposal of fixed assets		2,283	150
Amortization of convertible debt discount and debt issuance costs	4,826
Premium paid on securities purchased	(857)	(543)
Changes in operating assets and liabilities:
Accounts receivable	(410)	95
Inventory	(1,799)
Prepaid expenses, other current and long-term assets	387	(2,873)	(526)
Accounts payable	4,188	(993)	1,581
Accrued expenses and other current and long-term liabilities	9,432	2,766	429
Net cash used in operating activities	(107,831)	(62,472)	(38,949)
Cash flows from investing activities
Purchases of property and equipment	(2,146)	(8,440)	(5,197)
Change in restricted cash	(120)	(400)	48
Purchases of marketable securities	(356,754)	(196,061)	(145,798)
Sale and redemption of marketable securities	240,228	72,147	137,702
Net cash used in investing activities	(118,792)	(132,754)	(13,245)
Cash flows from financing activities
Proceeds from the issuance of 2024 convertible notes	201,250
Payment of debt issuance costs	(6,470)	(42)	(108)
Proceeds from the offering of common stock	132,666	147,889
Payments on notes payable	(8,333)		(3,500)
Proceeds from issuance of notes payable		15,000	15,004
Payments of public offering costs	(490)	(293)	(225)
Proceeds from the exercise of stock options	3,858	167	593
Proceeds from Employee Stock Purchase Plan	1,016	476	276
Net cash provided by financing activities	323,497	163,197	12,040
Net increase (decrease) in cash and cash equivalents	96,874	(32,029)	(40,154)
Cash and cash equivalents at beginning of period	30,915	62,944	103,098
Cash and cash equivalents at end of period	127,789	30,915	62,944
Supplemental disclosures of cash flow information:
Cash paid for interest	5,080	1,297	572
Supplemental disclosures of non-cash financing activities:
Public offering costs included in accounts payable or accrued		95
Portion of debt proceeds allocated to equity component	62,466
Purchases of property and equipment in accounts payable and accrued expenses	$ 9	$ 622	$ 1,576

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business	1. 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 biopharmaceutical 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®, as the first and only extended-release, intra-articular, or IA (meaning in the joint), injection indicated for the management of OA related knee pain. ZILRETTA is a non-opioid therapy that employs Flexion’s proprietary microsphere technology to provide pain relief over 12 weeks. 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. The accompanying 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 December 31, 2017, the Company had cash, cash equivalents, marketable securities, and long-term investments of

Financing Transactions_Common S

Financing Transactions/Common Stock	12 Months Ended
	Dec. 31, 2017
Common Stock [Member]
Financing Transactions/Common Stock	12. Common Stock On June 7, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,900,000 shares of the Company’s common stock at a price to the public of $14.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. On November 15, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 4,140,000 shares of the Company’s common stock at a price to the public of $18.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. 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 additional shares. 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 the preferential dividend rights of any holders of Preferred Stock. As of December 31, 2017, no dividends have been declared.
IPO [Member]
Financing Transactions/Common Stock	2. Financing Transactions On June 8, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 5,900,000 shares of the Company’s common stock at a price to the public of $14.00 per share including shares sold pursuant to the partial exercise of the underwriters’ option to purchase additional shares. The Company received net proceeds from the follow-on financing of $77.4 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. On November 15, 2016, the Company completed a follow-on public offering of its common stock, which resulted in the sale of 4,140,000 shares of the Company’s common stock at a price to the public of $18.00 per share including shares sold pursuant to the exercise in full of the underwriters’ option to purchase additional shares. The Company received net proceeds from the follow-on financing of $70.1 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. 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. 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 additional shares. The Company received net proceeds from the follow-on financing of $132.2 million after deducting underwriting discounts, commissions, and offering costs paid by the Company. The Company’s total issued common stock as of December 31, 2017 was 37,610,897 shares.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	3. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements 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 financial information, including the accounts of the Company and its wholly owned subsidiary after elimination of all significant intercompany accounts and transactions. 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 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. Revenue Recognition On October 6, 2017, the FDA approved ZILRETTA. The Company entered into a limited number of arrangements with specialty distributors and a specialty pharmacy in the U.S. (collectively, its “Customers”) to distribute ZILRETTA. These arrangements are the Company’s initial contracts with customers and, as a result the Company adopted Accounting Standards Codification (“ASC”) Topic 606 - Revenue from Contracts with Customers To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to arrangements that meet the definition of a contract with a customer under Topic 606, including when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For a complete discussion of accounting for product revenue, see Product Revenue, Net Product Revenue, Net — The Company sells ZILRETTA to its customers who then subsequently resell the Company’s products to physicians, clinics and certain medical centers or hospitals. In addition to distribution agreements with customers, the Company enters into arrangements with government payers that provide for government mandated rebates and chargebacks with respect to the purchase of the Company’s products. The Company recognizes revenue on product sales when the customer obtains control of the Company's product, which occurs at a point in time (upon delivery to the customer). We have determined that the delivery of ZILRETTA to our customers constitutes a single performance obligation. There are no other promises to deliver goods or services beyond what is specified in each accepted customer order. Management has assessed the existence of a significant financing component in the agreements with our customers. The trade payment terms with our customers do not exceed one year and therefore management has elected to apply the practical expedient and no amount of consideration has been allocated as a financing component. Product revenues are recorded net of applicable reserves for variable consideration, including discounts and allowances. Transaction Price, including Variable Consideration — Revenues from product sales are recorded at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established. Components of variable consideration include trade discounts and allowances, product returns, government chargebacks, discounts and rebates, and other incentives, such as voluntary patient assistance, and other fee for service amounts that are detailed within contracts between the Company and its customers relating to the Company’s sale of its products. These reserves, as detailed below, are based on the amounts earned, or to be claimed on the related sales, and are classified as reductions of accounts receivable (if the amount is payable to the customer) or a current liability (if the amount is payable to a party other than a customer). These estimates take into consideration a range of possible outcomes which are probability-weighted in accordance with the expected value method in Topic 606 for relevant factors such as current contractual and statutory requirements, specific known market events and trends, industry data, and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled based on the terms of the respective underlying contracts. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized under the contract will not occur in a future period. The Company’s analyses also contemplated application of the constraint in accordance with the guidance, under which it determined a material reversal of revenue would not occur in a future period for the estimates detailed below as of December 31, 2017 and, therefore, the transaction price was not reduced further during the year ended December 31, 2017. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s original estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such variances become known. Trade Discounts and Allowances —The Company compensates (through trade discounts and allowances) its customers for sales order management, data, and distribution services. However, the Company has determined such services received to date are not distinct from the Company’s sale of products to the customer and, therefore, these payments have been recorded as a reduction of revenue within the statement of operations and comprehensive loss through December 31, 2017, as well as a reduction to trade receivables, net on the condensed consolidated balance sheets. Product Returns — Consistent with industry practice, the Company generally offers customers a limited right of return for product that has been purchased from the Company based on the product’s expiration date. The Company estimates the amount of its product sales that may be returned by its customers and records this estimate as a reduction of revenue in the period the related product revenue is recognized, as well as within accrued expenses and other current liabilities, net on the condensed consolidated balance sheets. The Company currently estimates product return liabilities using available industry data and its own sales information, including its visibility into the inventory remaining in the distribution channel. The Company has not received any returns to date and believes that returns of its products will be minimal. Government Chargebacks, Discounts and Rebates — Chargebacks for fees and discounts to qualified government healthcare providers represent the estimated obligations resulting from contractual commitments to sell products to qualified VA hospitals and 340b entities at prices lower than the list prices charged to customers who directly purchase the product from the Company. The 340b Drug Discount Program is a US federal government program created in 1992 that requires drug manufacturers to provide outpatient drugs to eligible health care organizations and covered entities at significantly reduced prices. Customers charge the Company for the difference between what they pay for the product and the statutory selling price to the qualified government entity. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and trade receivables, net. Chargeback amounts are generally determined at the time of resale to the qualified government healthcare provider by customers, and the Company generally issues credits for such amounts within a few weeks of the Customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel inventories at each reporting period-end that the Company expects will be sold to qualified healthcare providers, and chargebacks that customers have claimed, but for which the Company has not yet issued a credit. Government Rebates — The Company is subject to discount obligations under state Medicaid programs and Medicare. These reserves are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included in accrued expenses and other current liabilities on the condensed consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. We anticipate the Company’s exposure to utilization from the Medicare Part D coverage gap discount program to be immaterial. For Medicaid programs, the Company estimates the portion of sales attributed to Medicaid patients and records a liability for the rebates to be paid to the respective state Medicaid programs. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but which remains in the distribution channel inventories at the end of each reporting period. Other Incentives — Other incentives which the Company offers include voluntary patient assistance programs, such as the co-pay assistance program, which are intended to provide financial assistance to qualified commercially-insured patients with prescription drug co-payments required by payers. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive associated with product that has been recognized as revenue, but remains in the distribution channel inventories at the end of each reporting period. The adjustments are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included as a component of accrued expenses and other current liabilities on the condensed consolidated balance sheets. To date, the Company’s only source of product revenue has been from the U.S. sales of ZILRETTA, which it began shipping to customers in October 2017. The following table summarizes activity in each of the product revenue allowance and reserve categories for the year ended December 31, 2017: (In thousands) Trade Discounts, Allowances and Government chargebacks Government rebates and other incentives Returns Total Beginning Balance $ — $ — $ — $ — Provision related to sales in the current year 100 15 2 117 Credit and payments made (40 ) 0 0 (40 ) Ending Balance $ 60 $ 15 $ 2 $ 77 Product Revenue Reserves and Allowances – Chargebacks and fees are recorded as reductions of trade receivables, net on the condensed consolidated balance sheets. Government and other rebates and returns are recorded as a component of accrued expenses and other current liabilities on the condensed consolidated balance sheets. Inventory —The Company values its inventories at the lower of cost or estimated net realizable value. The Company determines the cost of its inventories, which includes amounts related to materials and manufacturing overhead, on a first-in, first-out basis. The Company performs an assessment of the recoverability of capitalized inventory during each reporting period, and it writes down any excess and obsolete inventories to their estimated realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded within cost of product revenues. The determination of whether inventory costs will be realizable requires estimates by management. If actual market conditions are less favorable than projected by management, additional write-downs of inventory may be required, which would be recorded as a cost of product sales in the consolidated statements of operations and comprehensive loss. The Company capitalizes inventory costs associated with the Company’s products after regulatory approval when, based on management’s judgment, future commercialization is considered probable and the future economic benefit is expected to be realized. Inventory acquired prior to receipt of marketing approval of a product candidate is expensed as research and development expense as incurred. Inventory that can be used in either the production of clinical or commercial product is expensed as research and development expense when selected for use in a clinical manufacturing campaign. Inventory produced that will be used in promotional marketing campaigns is expensed to selling, general and administrative expense when it is selected for use in a marketing program. Shipping and handling costs for product shipments are recorded as incurred in cost of product revenues along with costs associated with manufacturing the product, and any inventory write-downs. Consolidation The accompanying consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions. In addition, Flexion Therapeutics, Inc. is registered to do business in the United Kingdom through its branch office located in Swindon, United Kingdom. U.S. Government Grant In 2015, the Company performed research and development for a U.S. Government agency under a cost reimbursable grant for clinical development of ZILRETTA. The related costs incurred under the grant are included in research and development expense in the statement of operations. Reimbursements were recorded as an offset to R&D expense when invoices for allowable costs were prepared and submitted to the Department of Defense. Due to challenges of enrolling military personnel with post-traumatic knee OA, we discontinued the Phase 2 trial and terminated the grant as of July 31, 2016. Payments under cost reimbursable grants with agencies of the U.S. Government are provisional payments subject to adjustment upon audit by the U.S. government. Accounts Receivable Accounts receivable are recorded net of customer allowances for distribution fees and chargebacks, and doubtful accounts. Allowances for distribution fees and chargebacks are based on contractual terms. The Company estimates the allowance for doubtful accounts based on existing contractual payment terms, actual payment patterns of its customers and individual customer circumstances. At December 31, 2017, the Company determined that an allowance for doubtful accounts was not required. No accounts were written off during the year ended December 31, 2017. Cash and Cash Equivalents The Company considers all highly liquid investments with a maturity of three months or less at the date of purchase to be cash equivalents. The Company currently invests available cash in money market funds of a major financial institution, corporate bonds, government obligations and commercial paper. Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is recorded in other income. Restricted Cash The balance at December 31, 2017 consists of $600,000 in an account at a commercial bank to collateralize a credit card account, which is classified as long-term restricted cash. The balance at December 31, 2016 consists of a $30,000 certificate of deposit to collateralize a credit card account with a commercial bank with an additional $400,000 to further collateralize the credit card account, and classified as long-term restricted cash. In addition, the Company held a letter of credit to the lessor of the Company’s Burlington facility of $50,000 as a security deposit pursuant to the lease agreement and classified as long-term restricted cash. In 2017, the certificate of deposit and the letter of credit were closed and reclassified out of restricted cash. 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 betterments 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, which is not yet in service, and is estimated to have a useful life of 7 years once placed into service. Foreign Currencies The Company maintains a bank account designated 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 within other income and expenses. Impairment of Long-Lived Assets The Company reviews its long-lived assets, including property and equipment, for impairment 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. Debt Issuance Costs, net As of December 31, 2017 and 2016, the carrying value of debt issuance costs was $4,111,729 and $100,760, respectively, presented as a direct deduction from the carrying amounts of long-term debt. In addition, $356,852, $36,607 and $41,103 respectively, of debt issuance costs were amortized and recognized as interest expense in the statement of operations for the years ended December 31, 2017, 2016 and 2015. Research and Development Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, depreciation, clinical trial and related clinical manufacturing costs, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred. Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are recorded as general and administrative expenses as incurred, as recoverability of such expenditures is uncertain. Accounting for Stock-Based Compensation The Company measures all stock options and other stock based-awards granted to employees at the fair value at the date of grant using the Black-Scholes option-pricing model. The fair value of the awards is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The straight-line method of expense recognition is applied to all awards with service-only conditions. For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is re-measured using the then current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified, or in the case of a non-employee, in the same manner as the award recipient’s service costs are classified. The Company recognizes compensation expense only for the portion of awards that are expected to vest. The Company accounts for forfeitures as they occur and does not estimate future forfeitures. As such, previously recognized compensation expense for an award shall be reversed in the period that the award is forfeited. Concentration of Credit Risk and Significant Suppliers Financial instruments that potentially expose the Company to concentration of credit risk consist primarily of commercial paper and corporate bonds. The Company generally invests its cash in money market funds, government and corporate bonds, and commercial paper at one financial institution. 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 is completely dependent on third-party manufacturers and product suppliers for research and commercial activities. In particular, the Company relies on a limited number of manufacturers and relies on them to purchase from third-party suppliers the materials necessary to produce its product candidates for its clinical trials and for commercial supply. These programs would be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients. For the year-ended December 31, 2017, two individual customers accounted for 94% of the Company’s total revenue and accounts receivable. No other customer accounted for more than 10% of net product revenue or accounts receivable for the year-ended December 31, 2017. Comprehensive Loss Comprehensive income (loss) includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) on available-for-sale securities. Income Taxes The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated 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 differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. 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 deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential 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 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 financial statements. The amount of 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. Fair Value Measurements Fair value is defined as the exchange price that would be received to sell 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 at 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 market prices in active markets for identical assets or liabilities. Level 1 consists primarily of financial instruments whose value is based on quoted market prices, such as exchange-traded instruments and listed equities. • 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 financial instruments consist of cash equivalents, marketable securities, restricted cash, accounts payable and accrued expenses, its term loan and 2024 Convertible Notes (Note 10). The estimated fair value of the Company’s financial instruments, with the exception of the 2024 Convertible Notes, approximates their carrying values. 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 $248.9 million at December 31, 2017. Net Loss Per Share The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based on their respective rights to receive dividends as if all income for the period had been distributed. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including the assumed conversion of our 2024 Convertible Notes, outstanding stock options and unvested restricted common stock, except where the result would be anti-dilutive. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of the conversion of the 2024 Convertible Notes, the exercise of outstanding stock options and the vesting unvested restricted common stock. In the diluted net loss per share calculation, net loss would also be adjusted for the elimination of interest expense on the 2024 Convertible Notes, if the impact was not anti-dilutive. For periods in which the Company has reported net losses, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Potential common shares will always be anti-dilutive for periods in which the Company has reported a net loss. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders for the years ended December 31, 2017, 2016 and 2015. Segment Data The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company is a biopharmaceutical company focused on the development and commercialization of novel, local therapies. All revenues for the year ended December 31, 2017 were generated in the United States. Recently Issued and Adopted Accounting Pronouncements 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 service

Fair Value of Financial Assets

Fair Value of Financial Assets	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets	4. Fair Value of Financial Assets The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2017 and 2016 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of December 31, 2017 Units: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 109,196 $ — $ 109,196 Marketable securities — 296,127 — 296,127 $ — $ 405,323 $ — $ 405,323 Fair Value Measurements as of December 31, 2016 Units: (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 December 31, 2017 and 2016, the Company’s cash equivalents that are invested in money market funds are valued based on Level 2 inputs. The Company measures the fair value of marketable securities using Level 2 inputs and primarily relies on quoted prices in active markets for similar marketable securities. During the years ended December 31, 2017 and 2016, there were no transfers between Level 1, Level 2 and Level 3. Amortization and accretion of discounts and premiums are recorded in other income. 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 December 31, 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 $248.9 million at December 31, 2017.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Marketable Securities	5. Marketable Securities As of December 31, 2017 and 2016, the fair value of available-for-sale marketable securities by type of security was as follows: December 31, 2017 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial Paper $ 22,436 $ — $ 22,436 U.S.Government obligations $ 121,470 $ — $ (136 ) $ 121,334 Corporate Bonds $ 152,630 $ — $ (273 ) $ 152,357 $ 296,536 $ — $ (409 ) $ 296,127 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 At December 31, 2017, marketable securities consisted of $264.6 million of investments that mature within twelve months and $31.5 million of investments that mature within fifteen months. At December 31, 2016, marketable securities consisted of $174.7 million of investments that mature within twelve months and $4.7 million of investments that mature within fifteen months.

Prepaid Expenses, Other Current

Prepaid Expenses, Other Current Assets, and Other Assets	12 Months Ended
	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid Expenses, Other Current Assets, and Other Assets	6. Prepaid Expenses, Other Current Assets, and Other Assets Prepaid expenses and other current assets and other assets consisted of the following as of December 31, 2017 and 2016: December 31, (in thousands) 2017 2016 Prepaid expenses $ 2,359 $ 1,086 Deposits 66 2,099 Interest receivable on marketable securities 978 605 Total prepaid expenses and other current assets $ 3,403 $ 3,790 On December 1, 2016, Flexion paid a refundable NDA fee in the amount of $2.0 million 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.

Inventory

Inventory	12 Months Ended
	Dec. 31, 2017
Inventory Disclosure [Abstract]
Inventory	7. Inventory Inventory consisted of the following as of December 31, 2017: December 31, (In thousands) 2017 2016 Raw Materials $ 928 $ - Work in process 746 - Finished goods 125 - Total inventories $ 1,799 $ - Inventory acquired prior to receipt of the marketing approval for ZILRETTA was expensed as research and development expense as incurred. The Company began to capitalize the costs associated with the production of ZILRETTA upon receipt of FDA approval of ZILRETTA on October 6, 2017. The Company reduces its inventory to net realizable value for potentially excess, dated or obsolete inventory based on an analysis of forecasted demand compared to quantities on hand and any firm purchase orders, as well as product shelf life. At December 31, 2017, the Company determined that no write-downs to inventory for potentially excess, dated or obsolete inventory were required.

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Property and Equipment, Net	8. Property and Equipment, Net Property and equipment, net, as of December 31, 2017 and 2016 consisted of the following: December 31, (In thousands) 2017 2016 Computer and office equipment $ 1,124 $ 573 Manufacturing equipment 11,780 10,099 Furniture and fixtures 456 402 Software 434 434 Leasehold improvements 474 278 Construction—in progress 305 1,254 14,573 13,040 Less: Accumulated depreciation (3,384 ) (1,376 ) Total property and equipment, net $ 11,189 $ 11,664 Depreciation expense for the years ended December 31, 2017, 2016 and 2015, was $2.0 million , $1.2 million, and $0.2 million, respectively. During the years ended December 31, 2017, 2016, and 2015, $0, $2.7 million, and $0.2 million of property and equipment was disposed of, resulting in a loss of $0, $2.3 million, and $0.1 million, respectively.

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities	9. Accrued Expenses and Other Current Liabilities Accrued expenses and other current liabilities at December 31, 2017 and 2016 consisted of the following: December 31, (In thousands) 2017 2016 Research and Development $ 1,023 $ 1,605 Payroll and other employee-related expenses 9,309 3,393 Professional services fees 2,591 927 Interest expense 1,249 161 Other 211 159 Total accrued expenses and other current liabilities $ 14,383 $ 6,245

Debt

Debt	12 Months Ended
	Dec. 31, 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 December 31, 2017, the Company was compliant with financial 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 December 31, 2017, the carrying value of the term loan was approximately $22.9 million, of which $10.0 million was due within 12 months and $12.9 million 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 December 31, 2017 and December 31, 2016. As of December 31, 2017, annual principal and interest payments due under the 2015 term loan are as follows: Year Aggregate Minimum Payments 2018 11,082 2019 10,449 2020 4,383 Total $ 25,914 Less interest (311 ) Less final payment (2,700 ) Total $ 22,903 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 in proceeds for the sale of the 2024 Convertible Notes, after deducting fees and expenses of $6.5 million. 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 Company’s 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 Company has separated the convertible debt into liability and equity components based on the fair value of a similar debt instrument excluding the embedded conversion option. 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 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 December 31, 2017: ( in thousands Gross proceeds $ 201,250 Portion of proceeds allocated to equity component (additional paid-in capital) (64,541 ) Debt issuance costs (6,470 ) Portion of issuance costs allocated to equity component (additional paid-in capital) 2,075 Amortization of debt discount and debt issuance costs 4,793 Carrying value 2024 Convertible Notes $ 137,107

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2017
Temporary Equity Disclosure [Abstract]
Preferred Stock	11. Preferred Stock On February 17, 2014, the Company filed an amended and restated Certificate of Incorporation (the “Restated Certificate”) in connection with the closing of the Company’s initial public offering. As of December 31, 2017, under the Restated Certificate, the Company is authorized to issue 10,000,000 shares of preferred stock with a par value of $0.001 per share.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 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 lease term. In July 2015, the Company amended the Lease to add approximately 4,700 square feet of additional office space, with the option to lease and additional 5,400 square feet in the same building in Burlington, Massachusetts. 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. This amendment also extended the term of the Lease through October 31, 2019. On September 30, 2015, the Company exercised its option for the additional 5,400 square feet of office space. 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. The Company incurred rent expense of $998,108, $668,350, and $373,202 for the years ended December 31, 2017, 2016 and 2015, respectively. Future minimum lease payments under the Company’s lease obligations are as follows: Year Aggregate Minimum Payments (in thousands) 2018 1,338 2019 1,491 2020 1,533 2021 1,576 2022 1,447 2023 1,203 Total $ 8,588 Manufacturing and Supply Agreement with Patheon U.K. Limited In July 2015, the Company and Patheon U.K. Limited (“Patheon”) entered into a Manufacturing and Supply Agreement (the “Manufacturing Agreement”) and Technical Transfer and Service Agreement (the “Technical Transfer Agreement”) for the manufacture of ZILRETTA, the Company’s lead program, which is an intra-articular (IA), extended-release steroid for the treatment of osteoarthritis. Patheon agreed in the Technical Transfer Agreement to undertake certain transfer activities and construction services needed to prepare Patheon’s United Kingdom facility for the commercial manufacture of ZILRETTA in dedicated manufacturing suites. The Company provided Patheon with certain equipment and materials necessary to manufacture ZILRETTA and pays Patheon a monthly fee for such activities and reimburse Patheon for certain material, equipment and miscellaneous expenses and additional services. The initial term of the Manufacturing Agreement is 10 years from approval by the U.S. Food and Drug Administration, or FDA, of the Patheon manufacturing suites for ZILRETTA. The Company pays a monthly base fee to Patheon for the operation of the manufacturing suites and a per product fee for each vial based upon a forecast of commercial demand. The Company also reimburses Patheon for purchases of materials and equipment made on its behalf, certain nominal expenses and additional services. The Manufacturing Agreement will remain in full effect unless and until it expires or is terminated. Upon termination of the Manufacturing Agreement (other than termination by Flexion in the event that Patheon does not meet the construction and manufacturing milestones or for a breach by Patheon), Flexion will be obligated to pay for the costs incurred by Patheon associated with the removal of our manufacturing equipment and for Patheon’s termination costs up to a capped amount. Future minimum payments under the Company’s agreed obligations are as follows: Year Ending December 31, 2018 6,891 2019 8,108 2020 8,108 2021 8,108 2022 8,108 2023 and thereafter 24,323 Total $ 63,646 Evonik Supply Agreement In November 2016, the Company entered into a Supply Agreement with Evonik Corporation (“Evonik”) for the purchase of PLGA which is used in the manufacturing of potential clinical and commercial supply of ZILRETTA. Pursuant to the Supply Agreement, Flexion is obligated to submit rolling monthly forecasts to Evonik for PLGA supply, a portion of which will constitute binding orders. In addition, Flexion agreed to certain minimum purchase requirements, which decrease over time, and which do not apply (i) during periods in which Evonik is in material breach of the Supply Agreement or is unable to perform its obligations due to a force majeure event, (ii) with respect to orders that Evonik is unable to supply in excess of binding orders, (iii) for orders Evonik is unable to timely deliver or does not deliver conforming product and provides a credit for such order, or (iv) during an uncured material quality failure by Evonik. Flexion agreed to purchase PLGA batches at a specified price per gram in U.S. dollars, subject to adjustment from time to time, including due to changes in price indices and in the event the initial term of the Supply Agreement is extended. The total term of the agreement is five years. Upon termination of the Supply Agreement (other than termination due to the bankruptcy of either Evonik or Flexion) Flexion is obligated to pay the costs associated with the binding supply forecast provided to Evonik. The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties. Southwest Research Institute License Agreement On July 25, 2014, the Company entered into an exclusive worldwide license agreement with Southwest Research Institute (“SwRI”) with respect to the use of SwRI’s proprietary microsphere manufacturing technologies for certain steroids formulated with PLGA, including ZILRETTA. Under the agreement, the Company paid an upfront fee of $120,000 to SwRI. In February 2017, Flexion executed an agreement with SwRI to transfer manufacturing equipment to SwRI in consideration for SwRI deeming the additional milestone payment to have been fully paid by Flexion. FX201 Related Agreement In December 2017, we entered into a definitive agreement with GeneQuine Biotherapeutics GmbH (“GeneQuine”) to acquire the global rights to FX201. As part of the asset purchase transaction with GeneQuine, we made an upfront payment to GeneQuine of $2 million. We may also be required to make additional milestone payments during the development of FX201, including up to $8.7 million through Phase 2 proof of concept (PoC) and, following successful PoC, up to an additional $54 million in development and global regulatory approval milestone payments. The transaction was accounted for as an asset acquisition, as it did not qualify as a business combination. The upfront fee was attributed to the intellectual property acquired, and recognized as research and development expense in December 2017 as the FX201 rights had not been commercially approved, and have no alternative future use. Future milestone payments earned prior to regulatory approval of FX201 would be recognized as research and development expense in the period when the milestone events become probable of being achieved. Future milestones earned upon regulatory approval would be recognized as an intangible asset and amortized to expense over its estimated life. As of December 31, 2017 none of the future milestone payments owed under the arrangement was probable of being achieved. As part of the transaction, we became the direct licensee of certain underlying Baylor College of Medicine (Baylor) patents and other proprietary rights related to FX201 for human applications. The Baylor license agreement grants us an exclusive, royalty-bearing, world-wide right and license (with a right to sublicense) for human applications under its patent and other proprietary rights directly related to FX201, with a similar non-exclusive license to certain Baylor intellectual property rights that are not specific to FX201. The license agreement with Baylor includes a low single-digit royalty on net sales of FX201 and requires us to use reasonable efforts to develop FX201 according to timelines set out in the license agreement. In December 2017, we also entered into a Master Production Services Agreement with SAFC Carlsbad, Inc., a part of MilliporeSigma, for the manufacturing of pre-clinical and initial clinical supplies of FX201.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	14. Stock-Based Compensation 2013 Equity Incentive Plan On January 27, 2014, the Company’s stockholders approved the 2013 Equity Incentive Plan (the “2013 Plan”), which became effective on February 11, 2014, the date of execution of the underwriting agreement pursuant to which the Company’s common stock was priced for its initial public offering. Prior to the effective date of the 2013 Plan, the Company granted stock-based awards pursuant to the 2009 Stock Incentive Plan (the “2009 Plan), which had similar features to the 2013 Plan. The 2013 Plan provides for the grant of incentive stock options (“ISOs”), non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance-based stock awards, and other forms of equity compensation. Initially, the maximum number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2013 Plan was 2,337,616, which is the sum of (i) 1,230,012 shares, plus (ii) the number of shares remaining available for grant under the 2009 Plan, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2009 Plan (such as upon the expiration or termination of a stock award prior to vesting). Additionally, the number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 and continuing through and including January 1, 2023, by 4% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2013 Plan is 4,684,989 shares. As of December 31, 2017, there were 344,296 options outstanding under the 2009 Plan. On September 11, 2017, the Company’s compensation committee approved an amendment to the 2013 Plan to reserve an additional 1,500,000 of the Company’s common stock to be used exclusively for grants of inducement awards to individuals who were not previously employees or non-employee directors of the Company (or following a bona fide period of non-employment with the Company). The Company currently grants stock-based awards pursuant to the 2013 Plan. As of December 31, 2017, 2,313,178 shares were available for future issuance under the 2013 Plan. Stock option vesting typically occurs over four years for employees and directors and is at the discretion of the board of directors. Options granted have a maximum term of up to 10 years. As of December 31, 2017, there were 3,455,669 options outstanding under the 2013 Plan. Stock Options During the years ended December 31, 2017, 2016 and 2015, the Company granted stock options for the purchase of 1,448,100, 1,816,575, and 657,250 shares of common stock, respectively, to certain employees and directors. The vesting conditions for most of these awards are time-based, and the awards typically vest 25% after one year and monthly thereafter for the next 36 months, except for annual option grants to non-employee directors of the Company whose initial grants vest 25% after one year and monthly thereafter for the next 24 months and whose annual grants vest in equal monthly installments during the 12-month period following the grant date, pursuant to the Company’s Non-Employee Director Compensation Policy. Awards typically expire after 10 years. Stock Option Valuation The fair value of each stock option grant is estimated on the date of grant using the Black-Scholes option-pricing model. The Company was a private company prior to 2014, and lacks company-specific historical information for a sufficient period of time or implied volatility information. Therefore, it estimates its expected stock volatility based on the historical volatility of the Company’s common stock since the Company’s initial public offering together with 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 traded stock price for a sufficient period of time. 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 years ended December 31, 2017, 2016 and 2015 is as follows: December 31, 2017 2016 2015 Risk-free interest rates 1.97%-2.29% 0.74-1.75% 1.49-1.92% Expected dividend yield 0.00% 0.00% 0.00% Expected term (in years) 6.0 5.6 6.0 Expected volatility 69.9%-72.8% 67.3-99.9% 76.4-83.9% The following table summarizes stock option activity for the year ended December 31, 2017: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2016 3,079 $ 14.84 Granted 1,448 22.29 Exercised (308 ) 12.25 Cancelled (419 ) 22.78 Outstanding as of December 31, 2017 3,800 $ 17.75 Options vested and expected to vest at December 31, 2017 3,800 $ 17.75 Options exercisable at December 31, 2017 1,689 $ 14.99 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 308,011, 30,195, and, 109,441 options were exercised during the years ended December 31, 2017, 2016 and 2015, respectively. The aggregate intrinsic value of stock options exercised was $2,947,768, $236,889, and $1,584,657 for the years ended December 31, 2017, 2016 and 2015, respectively. At December 31, 2017, 2016 and 2015 the Company had options for the purchase of 3,799,965, 3,079,175, and 1,657,225 shares of common stock outstanding, with a weighted average remaining contractual term of 8.0, 7.8, and 7.9 years, respectively, and with a weighted average exercise price of $17.75, $14.84, and $14.28 per share, respectively. At December 31, 2017, 2016 and 2015 there were options for the purchase of 1,688,652, 1,173,671, and 728,621 shares of common stock exercisable under these stock option awards, with a weighted average remaining contractual life of 6.8, 6.7, 6.9 years , respectively, and an aggregate intrinsic value of $16,985,787, $8,745,505, and 7,714,057, respectively. The weighted average grant date fair value of options granted during the years ended December 31, 2017, 2016 and 2015 was $14.33, $11.50, and $15.08, respectively. Restricted Stock Units On January 4, 2016, the Company granted 189,300 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 ZILRETTA is approved (the “Milestone”). The number of shares that vest varies based on the timing of achieving the Milestone. 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, subject to ongoing employment with the Company for a period of 2 years. The 122,800 shares had an approximate value of $2.2 million as of the original grant date of which $1.6 million was recognized in the fourth quarter of 2017 upon achieving the milestone and the remaining $0.6 million will be recognized over a period of two years. The following table summarizes the RSU activity for the year ended December 31, 2017: (In thousands, except per share amounts) Number of Shares Weighted Grant Date Fair Value Nonvested as of December 31, 2016 189 $ 15.77 Granted — — Cancelled (66 ) — Vested/Released (41 ) 16.43 Nonvested as of December 31, 2017 82 $ 16.43 Stock-based Compensation The Company recorded stock-based compensation expense related to stock options, restricted stock and shares purchased under the employee stock purchase plan for the years ended December 31, 2017, 2016 and 2015 as follows: Year Ended December 31, (In thousands) 2017 2016 2015 Research and development $ 3,979 $ 2,341 $ 1,412 Selling, general and administrative 7,563 4,429 3,171 $ 11,542 $ 6,770 $ 4,583 As of December 31, 2017, unrecognized stock-based compensation expense for stock options outstanding was $25.5 million which is expected to be recognized over a weighted average period of 2.8 years. As of December 31, 2017, unrecognized stock-based compensation expense for restricted stock units outstanding was $0.6 million which is expected to be recognized over a period of 2 years. Employee Stock Purchase Plan On January 27, 2014, the Company’s stockholders approved the Employee Stock Purchase Plan. A total of 209,102 shares of common stock were reserved for issuance under this plan. The Employee Stock Purchase Plan became effective on February 11, 2014, the date of execution of the underwriting agreement pursuant to which the Company’s common stock was priced for its initial public offering. During the year ended December 31, 2017 and 2016, 89,704 and 20,896 shares, respectively, were purchased by employees under the plan. Additionally, the number of shares of common stock reserved for issuance under the Employee Stock Purchase Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 and continuing through and including January 1, 2023, by 1% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the board of directors.

Net Loss Per Share

Net Loss Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	15. Net Loss Per Share Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2017, 2016 and 2015: Year ended December 31, (In thousands) 2017 2016 2015 Numerator: Net loss $ (137,481 ) $ (71,894 ) $ (46,315 ) Net loss: $ (137,481 ) $ (71,894 ) $ (46,315 ) Denominator: Weighted average common shares outstanding, basic and diluted 33,027 25,297 21,497 Net loss per share, basic and diluted $ (4.16 ) $ (2.84 ) $ (2.15 ) The following common stock equivalents were excluded from the calculation of diluted net loss per share as including them would have an anti-dilutive effect: Year ended December 31, 2017 2016 Shares issuable upon conversion of the 2024 convertible notes 5,017 — Stock Options 3,602 2,345 Restricted Stock Units 147 188 8,766 2,533

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	16. Income Taxes The Company has generated losses since inception. Accordingly, there is no tax provision or benefit for the years ended December 31, 2017, 2016, and 2015, respectively . 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 3.0 5.0 4.9 Federal and state research and development tax credits 0.9 2.7 4.7 Change in deferred tax asset valuation allowance (11.6 ) (40.0 ) (45.4 ) Tax law change (25.1 ) Other (1.2 ) (1.7 ) 1.8 Effective income tax rate — % — % — % The Company’s net deferred tax assets consisted of the following: December 31, 2017 2016 Net operating loss carryforwards $ 48,496 $ 36,880 Research and development tax credit carryforwards 7,725 7,078 Accruals and other temporary differences 3,578 4,897 Debt discount (14,630 ) 0 Capitalized research and development expenses, net 29,673 34,579 Total deferred tax assets 74,842 83,434 Valuation allowance (74,842 ) (83,434 ) Net deferred tax asset $ — $ — As of December 31, 2017, the Company had federal and state net operating loss carryforwards of approximately $190.1 million and $147.8 million, respectively, which begin to expire in 2029 for federal purposes and in 2030 for state purposes. In addition, the Company had federal and state research and development tax credit carryforwards of approximately $5.4 million and $2.9, respectively, available to reduce future tax liabilities, which begin to expire in 2029 for federal purposes and 2025 for state purposes. Management of the Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets, which are comprised principally of net operating loss carryforwards and capitalized research and development expenses. Management has considered the Company’s history of cumulative net losses incurred since inception, as well as its lack of commercialization of any products or generation of any revenue from product sales since inception, and determined that it is more likely than not that the Company will not realize the benefits of its deferred tax assets. As a result, a full valuation allowance has been established at December 31, 2017 and 2016. On December 22, 2017, President Trump signed into law the Tax Cuts and Jobs Act (the “Act”), a tax reform bill which, among other items, reduces the current corporate federal tax rate to 21% from 35%. The rate reduction is effective January 1, 2018. The Company concluded that the Act will cause our deferred tax assets to be revalued. As changes in tax laws or rates are enacted, deferred tax assets and liabilities are adjusted through income tax expense in the period of enactment. During the fourth quarter, we estimated the reduction in the value of our deferred tax assets to be $34.5 million as a result of the Act, which was offset by a corresponding change in the valuation allowance. The Securities and Exchange Commission issued Staff Accounting Bulletin No. 118 (“SAB 118”) on December 23, 2017. SAB 118 provides a one-year measurement period from a registrant’s reporting period that includes the Act’s enactment date to allow the registrant sufficient time to obtain, prepare and analyze information to complete the accounting required under ASC 740. The ultimate impact of the Act on our reported results in 2018 and beyond may differ from the estimates provided herein, possibly materially, due to, among other things, changes in interpretations and assumptions we have made, guidance that may be issued, and other actions we may take as a result of the Act, different from that presently contemplated. During year ended December 31, 2017, the Company recorded to equity a deferred tax liability relating to the discount on its 2024 Convertible Notes (see Note 10) with an equal and offsetting adjustment to the valuation allowance. Section 382 of the Internal Revenue Code of 1986, as amended (“Section 382”), contains rules that limit the ability of a company that undergoes an ownership change to utilize its net operating losses (“NOLs”) and tax credits existing as of the date of such ownership change. Under the rules, such an ownership change is generally any change in ownership of more than 50% of a company’s stock within a rolling three-year period. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company. During the quarter ended June 30, 2014, the Company completed a Section 382 study through February 11, 2014. The results of this study showed that as of February 11, 2014, one historical ownership change within the meaning of section 382 had occurred in 2009. As a result of this Section 382 limitation, approximately $0.3 million of NOLs will expire unutilized. Through the quarter ending December 31, 2017, the Company has completed periodic updates to its Section 382 study through October 31, 2017, which have indicated ownership changes within the meaning of Section 382 have occurred in December 2014 and June 2016, however, it is not anticipated that a portion of the Company’s NOLs will expire unutilized as a result of the Section 382 limitations arising from these ownership changes. Subsequent ownership changes defined by Section 382 may further limit the amount of NOL carryforwards that could be utilized annually to offset future taxable income. Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017 and 2016 were as follows: Year Ended December 31, 2017 2016 2015 Valuation allowance as of beginning of year $ (83,434 ) $ (54,773 ) $ (33,826 ) Decreases recorded as benefit to income tax provision 36,606 4,771 2,858 Decreases recorded as benefit to equity 24,537 — — Increases recorded to income tax provision (52,551 ) (33,432 ) (23,805 ) Valuation allowance as of end of year $ (74,842 ) $ (83,434 ) $ (54,773 ) In each reporting period, the Company considers whether a tax position of the Company is more likely than not to be sustained upon examination, including resolution of any related appeals of litigation processes, based on the technical merits of the position. For tax positions meeting the more likely than not threshold, the tax amount recognized in the financial statements is reduced by the largest benefit that has a greater than fifty percent likelihood of being realized upon the ultimate settlement with the relevant taxing authority. No liabilities for unrecognized tax benefits were recorded as of December 31, 2017 and 2016. 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 tax examinations. The Company’s tax years are still open under statute from 2013 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. The resolution of tax matters is not expected to have a material effect on the Company’s consolidated financial statements.

Quarterly Financial Data (unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	17. Quarterly Financial Data (unaudited) The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) 2017 2017 2017 2017 Revenues $ 0 $ 0 $ 0 $ 355 Operating expenses 23,782 26,902 31,221 48,132 Net loss (23,879 ) (28,880 ) (34,188 ) (50,534 ) Net loss per common share—basic and diluted $ (0.75 ) $ (0.91 ) $ (1.07 ) $ (1.38 ) Weighted average common shares—basic and diluted 31,704 31,826 31,931 36,644 Three Months Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) 2016 2016 2016 2016 Operating expenses $ 16,673 $ 14,120 $ 17,435 $ 21,552 Net loss (16,815 ) (14,185 ) (17,782 ) (23,112 ) Net loss per common share—basic and diluted $ (0.78 ) $ (0.63 ) $ (0.65 ) $ (0.79 ) Weighted average common shares—basic and diluted 21,570 22,666 27,524 29,347

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements 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 financial information, including the accounts of the Company and its wholly owned subsidiary after elimination of all significant intercompany accounts and transactions.
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 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.
Revenue Recognition	Revenue Recognition On October 6, 2017, the FDA approved ZILRETTA. The Company entered into a limited number of arrangements with specialty distributors and a specialty pharmacy in the U.S. (collectively, its “Customers”) to distribute ZILRETTA. These arrangements are the Company’s initial contracts with customers and, as a result the Company adopted Accounting Standards Codification (“ASC”) Topic 606 - Revenue from Contracts with Customers To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract, and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to arrangements that meet the definition of a contract with a customer under Topic 606, including when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of Topic 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied. For a complete discussion of accounting for product revenue, see Product Revenue, Net Product Revenue, Net — The Company sells ZILRETTA to its customers who then subsequently resell the Company’s products to physicians, clinics and certain medical centers or hospitals. In addition to distribution agreements with customers, the Company enters into arrangements with government payers that provide for government mandated rebates and chargebacks with respect to the purchase of the Company’s products. The Company recognizes revenue on product sales when the customer obtains control of the Company's product, which occurs at a point in time (upon delivery to the customer). We have determined that the delivery of ZILRETTA to our customers constitutes a single performance obligation. There are no other promises to deliver goods or services beyond what is specified in each accepted customer order. Management has assessed the existence of a significant financing component in the agreements with our customers. The trade payment terms with our customers do not exceed one year and therefore management has elected to apply the practical expedient and no amount of consideration has been allocated as a financing component. Product revenues are recorded net of applicable reserves for variable consideration, including discounts and allowances. Transaction Price, including Variable Consideration — Revenues from product sales are recorded at the net sales price (transaction price), which includes estimates of variable consideration for which reserves are established. Components of variable consideration include trade discounts and allowances, product returns, government chargebacks, discounts and rebates, and other incentives, such as voluntary patient assistance, and other fee for service amounts that are detailed within contracts between the Company and its customers relating to the Company’s sale of its products. These reserves, as detailed below, are based on the amounts earned, or to be claimed on the related sales, and are classified as reductions of accounts receivable (if the amount is payable to the customer) or a current liability (if the amount is payable to a party other than a customer). These estimates take into consideration a range of possible outcomes which are probability-weighted in accordance with the expected value method in Topic 606 for relevant factors such as current contractual and statutory requirements, specific known market events and trends, industry data, and forecasted customer buying and payment patterns. Overall, these reserves reflect the Company’s best estimates of the amount of consideration to which it is entitled based on the terms of the respective underlying contracts. The amount of variable consideration which is included in the transaction price may be constrained, and is included in the net sales price only to the extent that it is probable that a significant reversal in the amount of the cumulative revenue recognized under the contract will not occur in a future period. The Company’s analyses also contemplated application of the constraint in accordance with the guidance, under which it determined a material reversal of revenue would not occur in a future period for the estimates detailed below as of December 31, 2017 and, therefore, the transaction price was not reduced further during the year ended December 31, 2017. Actual amounts of consideration ultimately received may differ from the Company’s estimates. If actual results in the future vary from the Company’s original estimates, the Company will adjust these estimates, which would affect net product revenue and earnings in the period such variances become known. Trade Discounts and Allowances —The Company compensates (through trade discounts and allowances) its customers for sales order management, data, and distribution services. However, the Company has determined such services received to date are not distinct from the Company’s sale of products to the customer and, therefore, these payments have been recorded as a reduction of revenue within the statement of operations and comprehensive loss through December 31, 2017, as well as a reduction to trade receivables, net on the condensed consolidated balance sheets. Product Returns — Consistent with industry practice, the Company generally offers customers a limited right of return for product that has been purchased from the Company based on the product’s expiration date. The Company estimates the amount of its product sales that may be returned by its customers and records this estimate as a reduction of revenue in the period the related product revenue is recognized, as well as within accrued expenses and other current liabilities, net on the condensed consolidated balance sheets. The Company currently estimates product return liabilities using available industry data and its own sales information, including its visibility into the inventory remaining in the distribution channel. The Company has not received any returns to date and believes that returns of its products will be minimal. Government Chargebacks, Discounts and Rebates — Chargebacks for fees and discounts to qualified government healthcare providers represent the estimated obligations resulting from contractual commitments to sell products to qualified VA hospitals and 340b entities at prices lower than the list prices charged to customers who directly purchase the product from the Company. The 340b Drug Discount Program is a US federal government program created in 1992 that requires drug manufacturers to provide outpatient drugs to eligible health care organizations and covered entities at significantly reduced prices. Customers charge the Company for the difference between what they pay for the product and the statutory selling price to the qualified government entity. These reserves are established in the same period that the related revenue is recognized, resulting in a reduction of product revenue and trade receivables, net. Chargeback amounts are generally determined at the time of resale to the qualified government healthcare provider by customers, and the Company generally issues credits for such amounts within a few weeks of the Customer’s notification to the Company of the resale. Reserves for chargebacks consist of credits that the Company expects to issue for units that remain in the distribution channel inventories at each reporting period-end that the Company expects will be sold to qualified healthcare providers, and chargebacks that customers have claimed, but for which the Company has not yet issued a credit. Government Rebates — The Company is subject to discount obligations under state Medicaid programs and Medicare. These reserves are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included in accrued expenses and other current liabilities on the condensed consolidated balance sheets. For Medicare, the Company also estimates the number of patients in the prescription drug coverage gap for whom the Company will owe an additional liability under the Medicare Part D program. We anticipate the Company’s exposure to utilization from the Medicare Part D coverage gap discount program to be immaterial. For Medicaid programs, the Company estimates the portion of sales attributed to Medicaid patients and records a liability for the rebates to be paid to the respective state Medicaid programs. The Company’s liability for these rebates consists of invoices received for claims from prior quarters that have not been paid or for which an invoice has not yet been received, estimates of claims for the current quarter, and estimated future claims that will be made for product that has been recognized as revenue, but which remains in the distribution channel inventories at the end of each reporting period. Other Incentives — Other incentives which the Company offers include voluntary patient assistance programs, such as the co-pay assistance program, which are intended to provide financial assistance to qualified commercially-insured patients with prescription drug co-payments required by payers. The calculation of the accrual for co-pay assistance is based on an estimate of claims and the cost per claim that the Company expects to receive associated with product that has been recognized as revenue, but remains in the distribution channel inventories at the end of each reporting period. The adjustments are recorded in the same period the related revenue is recognized, resulting in a reduction of product revenue and the establishment of a current liability which is included as a component of accrued expenses and other current liabilities on the condensed consolidated balance sheets. To date, the Company’s only source of product revenue has been from the U.S. sales of ZILRETTA, which it began shipping to customers in October 2017. The following table summarizes activity in each of the product revenue allowance and reserve categories for the year ended December 31, 2017: (In thousands) Trade Discounts, Allowances and Government chargebacks Government rebates and other incentives Returns Total Beginning Balance $ — $ — $ — $ — Provision related to sales in the current year 100 15 2 117 Credit and payments made (40 ) 0 0 (40 ) Ending Balance $ 60 $ 15 $ 2 $ 77 Product Revenue Reserves and Allowances – Chargebacks and fees are recorded as reductions of trade receivables, net on the condensed consolidated balance sheets. Government and other rebates and returns are recorded as a component of accrued expenses and other current liabilities on the condensed consolidated balance sheets. Inventory —The Company values its inventories at the lower of cost or estimated net realizable value. The Company determines the cost of its inventories, which includes amounts related to materials and manufacturing overhead, on a first-in, first-out basis. The Company performs an assessment of the recoverability of capitalized inventory during each reporting period, and it writes down any excess and obsolete inventories to their estimated realizable value in the period in which the impairment is first identified. Such impairment charges, should they occur, are recorded within cost of product revenues. The determination of whether inventory costs will be realizable requires estimates by management. If actual market conditions are less favorable than projected by management, additional write-downs of inventory may be required, which would be recorded as a cost of product sales in the consolidated statements of operations and comprehensive loss. The Company capitalizes inventory costs associated with the Company’s products after regulatory approval when, based on management’s judgment, future commercialization is considered probable and the future economic benefit is expected to be realized. Inventory acquired prior to receipt of marketing approval of a product candidate is expensed as research and development expense as incurred. Inventory that can be used in either the production of clinical or commercial product is expensed as research and development expense when selected for use in a clinical manufacturing campaign. Inventory produced that will be used in promotional marketing campaigns is expensed to selling, general and administrative expense when it is selected for use in a marketing program. Shipping and handling costs for product shipments are recorded as incurred in cost of product revenues along with costs associated with manufacturing the product, and any inventory write-downs.
Consolidation	Consolidation The accompanying consolidated financial statements include the Company and its wholly-owned subsidiary, Flexion Securities Corporation, Inc. The Company has eliminated all intercompany transactions. In addition, Flexion Therapeutics, Inc. is registered to do business in the United Kingdom through its branch office located in Swindon, United Kingdom.
U.S. Government Grant	U.S. Government Grant In 2015, the Company performed research and development for a U.S. Government agency under a cost reimbursable grant for clinical development of ZILRETTA. The related costs incurred under the grant are included in research and development expense in the statement of operations. Reimbursements were recorded as an offset to R&D expense when invoices for allowable costs were prepared and submitted to the Department of Defense. Due to challenges of enrolling military personnel with post-traumatic knee OA, we discontinued the Phase 2 trial and terminated the grant as of July 31, 2016. Payments under cost reimbursable grants with agencies of the U.S. Government are provisional payments subject to adjustment upon audit by the U.S. government.
Accounts Receivable	Accounts Receivable Accounts receivable are recorded net of customer allowances for distribution fees and chargebacks, and doubtful accounts. Allowances for distribution fees and chargebacks are based on contractual terms. The Company estimates the allowance for doubtful accounts based on existing contractual payment terms, actual payment patterns of its customers and individual customer circumstances. At December 31, 2017, the Company determined that an allowance for doubtful accounts was not required. No accounts were written off during the year ended December 31, 2017.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with a maturity of three months or less at the date of purchase to be cash equivalents. The Company currently invests available cash in money market funds of a major financial institution, corporate bonds, government obligations and commercial paper.
Marketable Securities	Marketable Securities Marketable securities consist of investments with original maturities greater than ninety days and less than one year from the balance sheet date. Long-term investments consist of investments with maturities of greater than one year. The Company classifies all of its investments as available-for-sale securities. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Realized gains and losses are determined on a specific identification basis and are included in other income (loss). Amortization and accretion of discounts and premiums is recorded in other income.
Restricted Cash	Restricted Cash The balance at December 31, 2017 consists of $600,000 in an account at a commercial bank to collateralize a credit card account, which is classified as long-term restricted cash. The balance at December 31, 2016 consists of a $30,000 certificate of deposit to collateralize a credit card account with a commercial bank with an additional $400,000 to further collateralize the credit card account, and classified as long-term restricted cash. In addition, the Company held a letter of credit to the lessor of the Company’s Burlington facility of $50,000 as a security deposit pursuant to the lease agreement and classified as long-term restricted cash. In 2017, the certificate of deposit and the letter of credit were closed and reclassified out of restricted cash.
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 betterments 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, which is not yet in service, and is estimated to have a useful life of 7 years once placed into service.
Foreign Currencies	Foreign Currencies The Company maintains a bank account designated 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 within other income and expenses.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews its long-lived assets, including property and equipment, for impairment 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.
Debt Issuance Costs, net	Debt Issuance Costs, net As of December 31, 2017 and 2016, the carrying value of debt issuance costs was $4,111,729 and $100,760, respectively, presented as a direct deduction from the carrying amounts of long-term debt. In addition, $356,852, $36,607 and $41,103 respectively, of debt issuance costs were amortized and recognized as interest expense in the statement of operations for the years ended December 31, 2017, 2016 and 2015.
Research and Development	Research and Development Research and development expenses are comprised of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, depreciation, clinical trial and related clinical manufacturing costs, contract services and other related costs. Research and development costs are expensed to operations as the related obligation is incurred.
Patent Costs	Patent Costs All patent-related costs incurred in connection with filing and prosecuting patent applications are recorded as general and administrative expenses as incurred, as recoverability of such expenditures is uncertain.
Accounting for Stock-Based Compensation	Accounting for Stock-Based Compensation The Company measures all stock options and other stock based-awards granted to employees at the fair value at the date of grant using the Black-Scholes option-pricing model. The fair value of the awards is recognized as expense, net of estimated forfeitures, over the requisite service period, which is generally the vesting period of the respective award. The straight-line method of expense recognition is applied to all awards with service-only conditions. For stock-based awards granted to non-employees, compensation expense is recognized over the period during which services are rendered by such non-employees until completed. At the end of each financial reporting period prior to completion of the service, the fair value of these awards is re-measured using the then current fair value of the Company’s common stock and updated assumption inputs in the Black-Scholes option-pricing model. The Company classifies stock-based compensation expense in the consolidated statements of operations in the same manner in which the award recipient’s payroll costs are classified, or in the case of a non-employee, in the same manner as the award recipient’s service costs are classified. The Company recognizes compensation expense only for the portion of awards that are expected to vest. The Company accounts for forfeitures as they occur and does not estimate future forfeitures. As such, previously recognized compensation expense for an award shall be reversed in the period that the award is forfeited.
Concentration of Credit Risk and Significant Suppliers	Concentration of Credit Risk and Significant Suppliers Financial instruments that potentially expose the Company to concentration of credit risk consist primarily of commercial paper and corporate bonds. The Company generally invests its cash in money market funds, government and corporate bonds, and commercial paper at one financial institution. 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 is completely dependent on third-party manufacturers and product suppliers for research and commercial activities. In particular, the Company relies on a limited number of manufacturers and relies on them to purchase from third-party suppliers the materials necessary to produce its product candidates for its clinical trials and for commercial supply. These programs would be adversely affected by a significant interruption in the supply of active pharmaceutical ingredients. For the year-ended December 31, 2017, two individual customers accounted for 94% of the Company’s total revenue and accounts receivable. No other customer accounted for more than 10% of net product revenue or accounts receivable for the year-ended December 31, 2017.
Comprehensive Loss	Comprehensive Loss Comprehensive income (loss) includes net loss as well as other changes in stockholders’ deficit that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income (loss) in all periods presented was unrealized gains (losses) on available-for-sale securities.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are recognized for the estimated 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 differences between financial statement carrying amounts of existing assets and liabilities and their respective tax basis. Deferred tax assets and liabilities are measured using enacted rates in effect for the year in which these temporary differences are expected to be recovered or settled. 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 deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential 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 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 financial statements. The amount of 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.
Fair Value Measurements	Fair Value Measurements Fair value is defined as the exchange price that would be received to sell 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 at 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 market prices in active markets for identical assets or liabilities. Level 1 consists primarily of financial instruments whose value is based on quoted market prices, such as exchange-traded instruments and listed equities. • 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 financial instruments consist of cash equivalents, marketable securities, restricted cash, accounts payable and accrued expenses, its term loan and 2024 Convertible Notes (Note 10). The estimated fair value of the Company’s financial instruments, with the exception of the 2024 Convertible Notes, approximates their carrying values. 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 $248.9 million at December 31, 2017.
Net Loss Per Share	Net Loss Per Share The Company follows the two-class method when computing net loss per share as the Company has issued shares that meet the definition of participating securities. The two-class method determines net loss per share for each class of common and participating securities according to dividends declared or accumulated and participation rights in undistributed earnings. The two-class method requires income available to common stockholders for the period to be allocated between common and participating securities based on their respective rights to receive dividends as if all income for the period had been distributed. Basic net loss per share attributable to common stockholders is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding for the period. Diluted net loss attributable to common stockholders is computed by adjusting net loss attributable to common stockholders to reallocate undistributed earnings based on the potential impact of dilutive securities, including the assumed conversion of our 2024 Convertible Notes, outstanding stock options and unvested restricted common stock, except where the result would be anti-dilutive. Diluted net loss per share attributable to common stockholders is computed by dividing the diluted net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of the conversion of the 2024 Convertible Notes, the exercise of outstanding stock options and the vesting unvested restricted common stock. In the diluted net loss per share calculation, net loss would also be adjusted for the elimination of interest expense on the 2024 Convertible Notes, if the impact was not anti-dilutive. For periods in which the Company has reported net losses, diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders, since dilutive common shares are not assumed to have been issued if their effect is anti-dilutive. Potential common shares will always be anti-dilutive for periods in which the Company has reported a net loss. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders for the years ended December 31, 2017, 2016 and 2015.
Segment Data	Segment Data The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions. The Company is a biopharmaceutical company focused on the development and commercialization of novel, local therapies. All revenues for the year ended December 31, 2017 were generated in the United States.
Recently Issued and Adopted Accounting Pronouncements	Recently Issued and Adopted Accounting Pronouncements 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 to date, 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. As the Company had not recognized any revenue under the prior revenue recognition guidance, there was no impact from the adoption of the new revenue recognition guidance. In July 2015, the FASB issued ASU No. 2015-11, Inventory (Topic 330): Simplifying the Measurement of Inventory In November 2015, the FASB issued ASU 2015-17, Income Taxes In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities 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. ASU 2016-09 became effective for the Company on January 1, 2017. Upon adoption, the Company no longer records stock compensation expense net of forfeitures and the impact of adopting the guidance was not material to the consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments In August 2016, the FASB issued ASU 2016-15, Statement of cash flows (Topic 230), to increase the consistency of presentation in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. ASU 2016-15 will become effective for fiscal years, and the interim periods within those years, beginning after December 15, 2017. The Company is currently evaluating the potential impact that the adoption of this guidance may have on the Company’s financial statements. 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. In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Product Revenue Allowance And Reserve Categories	The following table summarizes activity in each of the product revenue allowance and reserve categories for the year ended December 31, 2017: (In thousands) Trade Discounts, Allowances and Government chargebacks Government rebates and other incentives Returns Total Beginning Balance $ — $ — $ — $ — Provision related to sales in the current year 100 15 2 117 Credit and payments made (40 ) 0 0 (40 ) Ending Balance $ 60 $ 15 $ 2 $ 77
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 Assets

Fair Value of Financial Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis	The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis as of December 31, 2017 and 2016 and indicate the level of the fair value hierarchy utilized to determine such fair value: Fair Value Measurements as of December 31, 2017 Units: (In thousands) Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ — $ 109,196 $ — $ 109,196 Marketable securities — 296,127 — 296,127 $ — $ 405,323 $ — $ 405,323 Fair Value Measurements as of December 31, 2016 Units: (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)	12 Months Ended
	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of December 31, 2017 and 2016, the fair value of available-for-sale marketable securities by type of security was as follows: December 31, 2017 (In thousands) Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Commercial Paper $ 22,436 $ — $ 22,436 U.S.Government obligations $ 121,470 $ — $ (136 ) $ 121,334 Corporate Bonds $ 152,630 $ — $ (273 ) $ 152,357 $ 296,536 $ — $ (409 ) $ 296,127 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

Prepaid Expenses, Other Curre28

Prepaid Expenses, Other Current Assets, and Other Assets (Tables)	12 Months Ended
	Dec. 31, 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 December 31, 2017 and 2016: December 31, (in thousands) 2017 2016 Prepaid expenses $ 2,359 $ 1,086 Deposits 66 2,099 Interest receivable on marketable securities 978 605 Total prepaid expenses and other current assets $ 3,403 $ 3,790

Inventory (Tables)

Inventory (Tables)	12 Months Ended
	Dec. 31, 2017
Inventory Disclosure [Abstract]
Schedule of Inventory	Inventory consisted of the following as of December 31, 2017: December 31, (In thousands) 2017 2016 Raw Materials $ 928 $ - Work in process 746 - Finished goods 125 - Total inventories $ 1,799 $ -

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment, net, as of December 31, 2017 and 2016 consisted of the following: December 31, (In thousands) 2017 2016 Computer and office equipment $ 1,124 $ 573 Manufacturing equipment 11,780 10,099 Furniture and fixtures 456 402 Software 434 434 Leasehold improvements 474 278 Construction—in progress 305 1,254 14,573 13,040 Less: Accumulated depreciation (3,384 ) (1,376 ) Total property and equipment, net $ 11,189 $ 11,664

Accrued Expenses and Other Cu31

Accrued Expenses and Other Current Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Payables And Accruals [Abstract]
Schedule of Accrued Expenses and Other Current Liabilities	Accrued expenses and other current liabilities at December 31, 2017 and 2016 consisted of the following: December 31, (In thousands) 2017 2016 Research and Development $ 1,023 $ 1,605 Payroll and other employee-related expenses 9,309 3,393 Professional services fees 2,591 927 Interest expense 1,249 161 Other 211 159 Total accrued expenses and other current liabilities $ 14,383 $ 6,245

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Annual Principal and Interest Payments Due Under Term Loan	As of December 31, 2017, annual principal and interest payments due under the 2015 term loan are as follows: Year Aggregate Minimum Payments 2018 11,082 2019 10,449 2020 4,383 Total $ 25,914 Less interest (311 ) Less final payment (2,700 ) Total $ 22,903
Summary of Carrying Value of Convertible Notes	The table below summarizes the carrying value of the 2024 Convertible Notes as of December 31, 2017: ( in thousands Gross proceeds $ 201,250 Portion of proceeds allocated to equity component (additional paid-in capital) (64,541 ) Debt issuance costs (6,470 ) Portion of issuance costs allocated to equity component (additional paid-in capital) 2,075 Amortization of debt discount and debt issuance costs 4,793 Carrying value 2024 Convertible Notes $ 137,107

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 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 (in thousands) 2018 1,338 2019 1,491 2020 1,533 2021 1,576 2022 1,447 2023 1,203 Total $ 8,588
Schedule of Future Minimum Payments under Contractual Obligations	Future minimum payments under the Company’s agreed obligations are as follows: Year Ending December 31, 2018 6,891 2019 8,108 2020 8,108 2021 8,108 2022 8,108 2023 and thereafter 24,323 Total $ 63,646

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 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 years ended December 31, 2017, 2016 and 2015 is as follows: December 31, 2017 2016 2015 Risk-free interest rates 1.97%-2.29% 0.74-1.75% 1.49-1.92% Expected dividend yield 0.00% 0.00% 0.00% Expected term (in years) 6.0 5.6 6.0 Expected volatility 69.9%-72.8% 67.3-99.9% 76.4-83.9%
Summary of Stock Option Activity	The following table summarizes stock option activity for the year ended December 31, 2017: (In thousands, except per share amounts) Shares Under Weighted Exercise Outstanding as of December 31, 2016 3,079 $ 14.84 Granted 1,448 22.29 Exercised (308 ) 12.25 Cancelled (419 ) 22.78 Outstanding as of December 31, 2017 3,800 $ 17.75 Options vested and expected to vest at December 31, 2017 3,800 $ 17.75 Options exercisable at December 31, 2017 1,689 $ 14.99
Summary of of RSU Activity	Restricted Stock Units On January 4, 2016, the Company granted 189,300 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 ZILRETTA is approved (the “Milestone”). The number of shares that vest varies based on the timing of achieving the Milestone. 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, subject to ongoing employment with the Company for a period of 2 years. The 122,800 shares had an approximate value of $2.2 million as of the original grant date of which $1.6 million was recognized in the fourth quarter of 2017 upon achieving the milestone and the remaining $0.6 million will be recognized over a period of two years. The following table summarizes the RSU activity for the year ended December 31, 2017: (In thousands, except per share amounts) Number of Shares Weighted Grant Date Fair Value Nonvested as of December 31, 2016 189 $ 15.77 Granted — — Cancelled (66 ) — Vested/Released (41 ) 16.43 Nonvested as of December 31, 2017 82 $ 16.43
Stock-Based Compensation Expense Related to Stock Options, Restricted Stock and Shares Purchased Under Employee Stock Purchase Plan	The Company recorded stock-based compensation expense related to stock options, restricted stock and shares purchased under the employee stock purchase plan for the years ended December 31, 2017, 2016 and 2015 as follows: Year Ended December 31, (In thousands) 2017 2016 2015 Research and development $ 3,979 $ 2,341 $ 1,412 Selling, general and administrative 7,563 4,429 3,171 $ 11,542 $ 6,770 $ 4,583

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders	Basic and diluted net loss per share attributable to common stockholders was calculated as follows for the years ended December 31, 2017, 2016 and 2015: Year ended December 31, (In thousands) 2017 2016 2015 Numerator: Net loss $ (137,481 ) $ (71,894 ) $ (46,315 ) Net loss: $ (137,481 ) $ (71,894 ) $ (46,315 ) Denominator: Weighted average common shares outstanding, basic and diluted 33,027 25,297 21,497 Net loss per share, basic and diluted $ (4.16 ) $ (2.84 ) $ (2.15 )
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 as including them would have an anti-dilutive effect: Year ended December 31, 2017 2016 Shares issuable upon conversion of the 2024 convertible notes 5,017 — Stock Options 3,602 2,345 Restricted Stock Units 147 188 8,766 2,533

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate	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 3.0 5.0 4.9 Federal and state research and development tax credits 0.9 2.7 4.7 Change in deferred tax asset valuation allowance (11.6 ) (40.0 ) (45.4 ) Tax law change (25.1 ) Other (1.2 ) (1.7 ) 1.8 Effective income tax rate — % — % — %
Net Deferred Tax Assets	The Company’s net deferred tax assets consisted of the following: December 31, 2017 2016 Net operating loss carryforwards $ 48,496 $ 36,880 Research and development tax credit carryforwards 7,725 7,078 Accruals and other temporary differences 3,578 4,897 Debt discount (14,630 ) 0 Capitalized research and development expenses, net 29,673 34,579 Total deferred tax assets 74,842 83,434 Valuation allowance (74,842 ) (83,434 ) Net deferred tax asset $ — $ —
Changes in Valuation Allowance for Deferred Tax Assets	Changes in the valuation allowance for deferred tax assets during the years ended December 31, 2017 and 2016 were as follows: Year Ended December 31, 2017 2016 2015 Valuation allowance as of beginning of year $ (83,434 ) $ (54,773 ) $ (33,826 ) Decreases recorded as benefit to income tax provision 36,606 4,771 2,858 Decreases recorded as benefit to equity 24,537 — — Increases recorded to income tax provision (52,551 ) (33,432 ) (23,805 ) Valuation allowance as of end of year $ (74,842 ) $ (83,434 ) $ (54,773 )

Quarterly Financial Data (una37

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Recurring Adjustments Necessary for Fair Statement	The following information has been derived from unaudited consolidated financial statements that, in the opinion of management, include all recurring adjustments necessary for a fair statement of such information. Three Months Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) 2017 2017 2017 2017 Revenues $ 0 $ 0 $ 0 $ 355 Operating expenses 23,782 26,902 31,221 48,132 Net loss (23,879 ) (28,880 ) (34,188 ) (50,534 ) Net loss per common share—basic and diluted $ (0.75 ) $ (0.91 ) $ (1.07 ) $ (1.38 ) Weighted average common shares—basic and diluted 31,704 31,826 31,931 36,644 Three Months Ended March 31, June 30, September 30, December 31, (in thousands, except per share amounts) 2016 2016 2016 2016 Operating expenses $ 16,673 $ 14,120 $ 17,435 $ 21,552 Net loss (16,815 ) (14,185 ) (17,782 ) (23,112 ) Net loss per common share—basic and diluted $ (0.78 ) $ (0.63 ) $ (0.65 ) $ (0.79 ) Weighted average common shares—basic and diluted 21,570 22,666 27,524 29,347

Nature of the Business - Additi

Nature of the Business - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Incorporation date	Nov. 5, 2007
Cash and cash equivalents and marketable securities	$ 423.9

Financing Transactions - Additi

Financing Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Oct. 16, 2017	May 02, 2017	Nov. 15, 2016	Jun. 08, 2016	Jun. 07, 2016	Dec. 31, 2017	Dec. 31, 2016
Class Of Stock [Line Items]
Net proceeds from follow-on public offering						$ 132,666	$ 147,889
Common stock, shares issued						37,610,897	31,667,469
2024 Convertible Notes [Member]
Class Of Stock [Line Items]
Debt instrument principal amount		$ 201,300
Debt instrument maturity date		May 1, 2024
Debt instrument interest rate		3.375%				3.375%
Debt instrument frequency of periodic payment						semi-annually
Debt instrument date of first required payment		Nov. 1, 2017
Net proceeds from offering of convertible senior notes		$ 194,800
Debt issuance costs		$ 6,500				$ 68,900
Follow On Public Offering [Member]
Class Of Stock [Line Items]
Common stock, shares issued	5,520,000		4,140,000	5,900,000	5,900,000
Sale of common stock price per share	$ 25.50		$ 18	$ 14	$ 14
Net proceeds from follow-on public offering	$ 132,200		$ 70,100	$ 77,400

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Customer	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	May 02, 2017USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accounts written off	$ 0
Long-term restricted cash	600,000	$ 480,000
Carrying value of debt issuance costs reported in prepaid expenses and other current assets	4,111,729	100,760
Amortization of debt issuance costs recognized as interest expense	$ 356,852	36,607	$ 41,103
Number of major customers | Customer	2
Number of customers accounted for more than 10% of accounts receivable | Customer	0
Number of customers accounted for more than 10% of net product revenue | Customer	0
2024 Convertible Notes [Member]
Summary Of Significant Accounting Policies [Line Items]
Convertible Notes, face value				$ 201,300,000
Estimated fair value of Convertible Notes	$ 248,900,000
Customer Concentration Risk | Accounts Receivable [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	94.00%
Customer Concentration Risk | Total Revenue [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage	94.00%
Construction-in Progress [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful life	7 years
Letter of Credit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash		50,000
Certificates of Deposit [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash		30,000
Commercial Bank [Member]
Summary Of Significant Accounting Policies [Line Items]
Long-term restricted cash	$ 600,000	$ 400,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Trade Payment Term	1 year

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Summary of Product Revenue Allowance And Reserve Categories (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Trade Discounts, Allowances and Government Chargebacks [Member]
Summary Of Significant Accounting Policies [Line Items]
Provision related to sales in the current year	$ 100
Credit and payments made	(40)
Ending Balance	60
Government Rebates and Other Incentives [Member]
Summary Of Significant Accounting Policies [Line Items]
Provision related to sales in the current year	15
Credit and payments made	0
Ending Balance	15
Returns [Member]
Summary Of Significant Accounting Policies [Line Items]
Provision related to sales in the current year	2
Credit and payments made	0
Ending Balance	2
Product Revenue Allowance and Reserve [Member]
Summary Of Significant Accounting Policies [Line Items]
Provision related to sales in the current year	117
Credit and payments made	(40)
Ending Balance	$ 77

Summary of Significant Accoun42

Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 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 Asset43

Fair Value of Financial Assets - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 109,196	$ 9,830
Marketable securities	296,127	179,414
Assets, Total	405,323	189,244
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	109,196	9,830
Marketable securities	296,127	179,414
Assets, Total	$ 405,323	$ 189,244

Fair Value of Financial Asset44

Fair Value of Financial Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 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	$ 248,900,000

Marketable Securities - Summary

Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 296,536	$ 179,486
Gross Unrealized Gains		6
Gross Unrealized Losses	(409)	(78)
Fair Value	296,127	179,414
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,436	7,769
Fair Value	22,436	7,769
U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,470	75,524
Gross Unrealized Gains		5
Gross Unrealized Losses	(136)	(12)
Fair Value	121,334	75,517
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	152,630	96,193
Gross Unrealized Gains		1
Gross Unrealized Losses	(273)	(66)
Fair Value	$ 152,357	$ 96,128

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Marketable securities	$ 264,589	$ 174,688
Marketable securities, noncurrent	$ 31,500	$ 4,700

Prepaid Expenses, Other Curre47

Prepaid Expenses, Other Current Assets, and Other Assets - Prepaid Expenses and Other Current Assets and Other Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 2,359	$ 1,086
Deposits	66	2,099
Interest receivable on marketable securities	978	605
Total prepaid expenses and other current assets	$ 3,403	$ 3,790

Prepaid Expenses, Other Curre48

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

Inventory - Schedule of Invento

Inventory - Schedule of Inventory (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Inventory Disclosure [Abstract]
Raw Materials	$ 928
Work in process	746
Finished goods	125
Total inventories	$ 1,799

Inventory - Additional Informat

Inventory - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)
Inventory Disclosure [Abstract]
Write-downs to inventory	$ 0

Property and Equipment, Net - C

Property and Equipment, Net - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment gross	$ 14,573	$ 13,040
Less: Accumulated depreciation	(3,384)	(1,376)
Total property and equipment, net	11,189	11,664
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	1,124	573
Manufacturing Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	11,780	10,099
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	456	402
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	474	278
Construction-in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment gross	305	1,254
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 ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation	$ 2,008	$ 1,151	$ 238
Property and equipment disposals	0	2,700	200
Gain (Loss) on sale of property and equipment	$ 0	$ (2,300)	$ (100)

Accrued Expenses and Other Cu53

Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Research and Development	$ 1,023	$ 1,605
Payroll and other employee-related expenses	9,309	3,393
Professional services fees	2,591	927
Interest expense	1,249	161
Other	211	159
Total accrued expenses and other current liabilities	$ 14,383	$ 6,245

Debt - Additional Information (

Debt - Additional Information (Detail)	May 02, 2017USD ($)	Jul. 22, 2016USD ($)	Aug. 04, 2015USD ($)	Dec. 31, 2017USD ($)d$ / sharesshares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Debt Instrument [Line Items]
Term loan due within 12 months				$ 9,967,000	$ 9,134,000
Term loan due in greater than 12 months				12,936,000	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	$ 108,000
Amortization of debt discount				333,000	$ 729,000	$ 871,000
2015 Term Loan [Member]
Debt Instrument [Line Items]
Carrying value of term loan				22,903
Liability component of Convertible Notes recorded as long-term debt				$ 25,914
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, payment description				Interest-only period of 19 months, principal will be due in 36 equal monthly installments.
Term loan, first periodic payment date			Mar. 1, 2017
Debt instrument maturity date			Feb. 1, 2020
Interest on final payment			9.00%
Carrying value of term loan				$ 22,900,000
Term loan due within 12 months				10,000,000
Term loan due in greater than 12 months				$ 12,900,000
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%
Term loan, first periodic payment date	Nov. 1, 2017
Debt instrument maturity date	May 1, 2024
Interest on final payment	3.375%			9.71%
Debt issuance costs	$ 6,500,000			$ 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
Debt instrument convertible initial conversion price per share of common stock | $ / shares				$ 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				9,000,000
Amortization of debt discount				$ 4,400,000
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]	Dec. 31, 2017USD ($)
Long-term Debt, Fiscal Year Maturity [Abstract]
2,018	$ 11,082
2,019	10,449
2,020	4,383
Total	25,914
Less interest	(311)
Less final payment	(2,700)
Total	$ 22,903

Debt - Summary of Carrying Valu

Debt - Summary of Carrying Value of Convertible Notes (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
Gross proceeds	$ 201,250
Amortization of debt discount and debt issuance costs	4,826
Carrying value 2024 Convertible Notes	137,107
2024 Convertible Notes [Member]
Debt Instrument [Line Items]
Gross proceeds	201,250
Gross Proceeds, 'Portion of proceeds allocated to equity component (additional paid-in capital)	(64,541)
Debt issuance costs	(6,470)
Debt issuance costs, 'Portion of issuance costs allocated to equity component (additional paid-in capital)	2,075
Amortization of debt discount and debt issuance costs	4,793
Carrying value 2024 Convertible Notes	$ 137,107

Preferred Stock - Additional In

Preferred Stock - Additional Information (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity Disclosure [Abstract]
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, par value	$ 0.001	$ 0.001

Common Stock - Additional infor

Common Stock - Additional information (Detail) - USD ($)	Oct. 16, 2017	Nov. 15, 2016	Jun. 08, 2016	Jun. 07, 2016	Dec. 31, 2017
Class Of Stock [Line Items]
Number of vote per 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.
Dividend declared					$ 0
Follow On Public Offering [Member]
Class Of Stock [Line Items]
Common stock, shares issued	5,520,000	4,140,000	5,900,000	5,900,000
Sale of common stock price per share	$ 25.50	$ 18	$ 14	$ 14

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)

May 01, 2018ft²

Oct. 06, 2017ft²

Apr. 07, 2017USD ($)ft²

Sep. 21, 2016ft²

Jul. 25, 2014USD ($)

Dec. 31, 2017USD ($)

Feb. 28, 2017USD ($)

Nov. 30, 2016Term

Jul. 31, 2015ft²

May 31, 2013USD ($)

Dec. 31, 2017USD ($)

Oct. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Sep. 30, 2015ft²

Commitment And Contingencies [Line Items]

Operating lease term

42 months

Minimum monthly lease payments

$ 17,588

Rent expense

$ 998,108

$ 668,350

$ 373,202

GeneQuine Biotherapeutics GmbH (“GeneQuine”) [Member] | Definitive Agreement [Member]

Commitment And Contingencies [Line Items]

License agreement, upfront fee paid

$ 2,000,000

Future milestone payments owed

0

$ 0

GeneQuine Biotherapeutics GmbH (“GeneQuine”) [Member] | Definitive Agreement [Member] | Maximum [Member]

Commitment And Contingencies [Line Items]

Additional milestone payments

54,000,000

Zilretta [Member] | Southwest Research Institute (SwRI) [Member] | Upon FDA First Approval [Member]

Commitment And Contingencies [Line Items]

License agreement, upfront fee paid

$ 120,000

Evonik Corporation [Member]

Commitment And Contingencies [Line Items]

Supply agreement period

5 years

Supply agreement renewal term

2 years

Number of renewal terms | Term

2

Supply agreement description

The Supply Agreement will renew for two successive two year terms upon mutual written consent by both parties.

Manufacturing and Supply Agreement with Patheon U.K. Limited [Member]

Commitment And Contingencies [Line Items]

Manufacturing agreement period

10 years

Phase 2 Proof of Concept (PoC) [Member] | GeneQuine Biotherapeutics GmbH (“GeneQuine”) [Member] | Definitive Agreement [Member] | Maximum [Member]

Commitment And Contingencies [Line Items]

Additional milestone payments

$ 8,700,000

Burlington Massachusetts [Member]

Commitment And Contingencies [Line Items]

Additional leased office space | ft²

6,450

1,471

4,700

Extended leased office space | ft²

5,400

5,400

Office space leased on temporary agreement | ft²

6,700

6,700

Lease termination date

Oct. 31, 2023

Oct. 31, 2019

Temporary lease termination date

Oct. 31, 2017

Office space leased | ft²

28,600

Lease payment per month for office space

$ 80,000

Lease payment per month for additional office space

$ 4,100

Operating lease expected commencement date

Apr. 1, 2018

Burlington Massachusetts [Member] | Scenario, Forecast [Member]

Commitment And Contingencies [Line Items]

Lease termination date

Oct. 31, 2023

Operating lease office space | ft²

36,500

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 Contingencies60

Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Commitments And Contingencies Disclosure [Abstract]
2,018	$ 1,338
2,019	1,491
2,020	1,533
2,021	1,576
2,022	1,447
2,023	1,203
Total	$ 8,588

Commitments and Contingencies61

Commitments and Contingencies - Schedule of Future Minimum Payments under Contractual Obligations (Detail) - Manufacturing and Supply Agreement with Patheon U.K. Limited [Member]	Dec. 31, 2017USD ($)
Property Subject To Or Available For Operating Lease [Line Items]
2,018	$ 6,891
2,019	8,108
2,020	8,108
2,021	8,108
2,022	8,108
2023 and thereafter	24,323
Total	$ 63,646

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($)	Oct. 06, 2017	Sep. 11, 2017	Jan. 04, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 27, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding				3,799,965	3,799,965	3,079,175	1,657,225
Stock options granted					1,448,000
Number of options exercised					308,011	30,195	109,441
Aggregate intrinsic value of stock options exercised					$ 2,947,768	$ 236,889	$ 1,584,657
Weighted average remaining contractual term					8 years	7 years 9 months 18 days	7 years 10 months 24 days
Weighted average exercise price				$ 17.75	$ 17.75	$ 14.84	$ 14.28
Options exercisable, shares				1,688,652	1,688,652	1,173,671	728,621
Weighted average remaining contractual term					6 years 9 months 18 days	6 years 8 months 12 days	6 years 10 months 24 days
Aggregate intrinsic value, stock options exercisable				$ 16,985,787	$ 16,985,787	$ 8,745,505	$ 7,714,057
Weighted average grant date fair value of options granted					$ 14.33	$ 11.50	$ 15.08
Unrecognized stock-based compensation expense				25,500,000	$ 25,500,000
Unrecognized compensation costs, weighted-average recognition periods					2 years 9 months 18 days
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period					4 years
Share based award, expiration period					10 years
Stock options granted					1,448,100	1,816,575	657,250
Awards vesting right					The vesting conditions for most of these awards are time-based, and the awards typically vest 25% after one year and monthly thereafter for the next 36 months
Employee Stock Option [Member] | Non Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards vesting right					Annual option grants to non-employee directors of the Company whose initial grants vest 25% after one year and monthly thereafter for the next 24 months
Employee Stock Option [Member] | Vesting after One Year and Monthly Thereafter for the Next 36 Months [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of share based award, vested					25.00%
Employee Stock Option [Member] | Vesting after One Year and Monthly Thereafter for the Next 24 Months [Member] | Non Employee Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of share based award, vested					25.00%
Restricted Stock Units (RSUs) [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		189,300
Grant date fair value of restricted common stock units under performance-based vesting	$ 2,200,000			1,600,000
Unrecognized stock-based compensation expense				$ 600,000	$ 600,000
Unrecognized compensation costs, weighted-average recognition periods					2 years
2013 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of aggregate number of shares common stock that may be issued					Initially, the maximum number of shares of the Company’s common stock that may be issued pursuant to stock awards under the 2013 Plan was 2,337,616, which is the sum of (i) 1,230,012 shares, plus (ii) the number of shares remaining available for grant under the 2009 Plan, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2009 Plan (such as upon the expiration or termination of a stock award prior to vesting).
Share based compensation, shares issued in period				1,230,012	1,230,012
Percentage of outstanding shares of common stock					4.00%
Options to purchase common stock, outstanding				3,455,669	3,455,669
Additional shares of common stock reserved		1,500,000
Number of shares available for future issuance				2,313,178	2,313,178
2009 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options to purchase common stock, outstanding				344,296	344,296
Employees Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of outstanding shares of common stock					1.00%
Number of shares available for future issuance								209,102
Number of shares purchased by employees					89,704	20,896
Maximum [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based award, expiration period					10 years
Maximum [Member] | 2013 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based compensation, authorized				2,337,616	2,337,616
Number of options exercised				4,684,989	4,684,989

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rates, Minimum	1.97%	0.74%	1.49%
Risk-free interest rates, Maximum	2.29%	1.75%	1.92%
Expected dividend yield	0.00%	0.00%	0.00%
Expected term (in years)	6 years	5 years 7 months 6 days	6 years
Expected volatility, Minimum	69.90%	67.30%	76.40%
Expected volatility, Maximum	72.80%	99.90%	83.90%

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Issuable Under Options, Beginning balance	3,079,175	1,657,225
Shares Issuable Under Options, Granted	1,448,000
Shares Issuable Under Options, Exercised	(308,011)	(30,195)	(109,441)
Shares Issuable Under Options, Cancelled	(419,000)
Shares Issuable Under Options, Ending balance	3,799,965	3,079,175	1,657,225
Shares Issuable Under Options, Options vested and expected to vest at December 31, 2017	3,800,000
Shares Issuable Under Options, Options exercisable at December 31, 2017	1,688,652	1,173,671	728,621
Weighted Average Exercise Price, Beginning balance	$ 14.84	$ 14.28
Weighted Average Exercise Price, Granted	22.29
Weighted Average Exercise Price, Exercised	12.25
Weighted Average Exercise Price, Cancelled	22.78
Weighted Average Exercise Price, Ending balance	17.75	$ 14.84	$ 14.28
Weighted Average Exercise Price, Options vested and expected to vest	17.75
Weighted Average Exercise Price, Options exercisable	$ 14.99

Stock-Based Compensation - Su65

Stock-Based Compensation - Summary of of RSU Activity (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares	Oct. 06, 2017	Jan. 04, 2016	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Shares, Beginning balance			189,000
Number of Shares, Granted	122,800	189,300
Number of Shares, Cancelled			(66,000)
Number of Shares, Vested/Released			(41,000)
Number of Shares, Ending balance			82,000
Weighted Average Grant Date Fair Value, Beginning balance			$ 15.77
Weighted Average Grant Date Fair Value, Vested/Released			16.43
Weighted Average Grant Date Fair Value, Ending balance			$ 16.43

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options, Restricted Stock and Shares Purchased Under Employee Stock Purchase Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 11,542	$ 6,770	$ 4,583
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	3,979	2,341	1,412
Selling, General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 7,563	$ 4,429	$ 3,171

Net Loss Per Share - Schedule o

Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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:
Net loss	$ (50,534)	$ (34,188)	$ (28,880)	$ (23,879)	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (137,481)	$ (71,894)	$ (46,315)
Net loss:	$ (50,534)	$ (34,188)	$ (28,880)	$ (23,879)	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (137,481)	$ (71,894)	$ (46,315)
Denominator:
Weighted average common shares outstanding, basic and diluted	36,644	31,931	31,826	31,704	29,347	27,524	22,666	21,570	33,027	25,297	21,497
Net loss per share, basic and diluted	$ (1.38)	$ (1.07)	$ (0.91)	$ (0.75)	$ (0.79)	$ (0.65)	$ (0.63)	$ (0.78)	$ (4.16)	$ (2.84)	$ (2.15)

Net Loss per Share - Schedule68

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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 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	8,766	2,533
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	5,017
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	3,602	2,345
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	147	188

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Tax provision or benefit			$ 0	$ 0	$ 0
Net operating loss carryforwards	$ 300,000		$ 300,000
Corporate federal tax rate			34.00%	34.00%	34.00%
Reduction of deferred tax assets	34,500,000
Limit of utilization of net operating losses (NOLs) and tax credits description			Under the rules, such an ownership change is generally any change in ownership of more than 50% of a company’s stock within a rolling three-year period. The rules generally operate by focusing on changes in ownership among stockholders considered by the rules as owning, directly or indirectly, 5% or more of the stock of a company and any change in ownership arising from new issuances of stock by the company.
Liabilities for unrecognized tax benefits	0		$ 0	$ 0
Maximum [Member]
Income Tax [Line Items]
Corporate federal tax rate			35.00%
Scenario, Plan [Member]
Income Tax [Line Items]
Corporate federal tax rate		21.00%
Federal [Member]
Income Tax [Line Items]
Net operating loss carryforwards	190,100,000		$ 190,100,000
Operating loss carryforwards expiration year			2,029
Federal [Member] | Research and Development Tax Credit Carryforwards [Member]
Income Tax [Line Items]
Tax credit carryforwards	5,400,000		$ 5,400,000
Tax credit expiration year			2,029
State and Local Jurisdiction [Member]
Income Tax [Line Items]
Net operating loss carryforwards	147,800,000		$ 147,800,000
Operating loss carryforwards expiration year			2,030
State and Local Jurisdiction [Member] | Research and Development Tax Credit Carryforwards [Member]
Income Tax [Line Items]
Tax credit carryforwards	$ 2,900,000		$ 2,900,000
Tax credit expiration year			2,025

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	34.00%	34.00%	34.00%
State taxes, net of federal benefit	3.00%	5.00%	4.90%
Federal and state research and development tax credits	0.90%	2.70%	4.70%
Change in deferred tax asset valuation allowance	(11.60%)	(40.00%)	(45.40%)
Tax law change	(25.10%)	0.00%	0.00%
Other	(1.20%)	(1.70%)	1.80%
Effective income tax rate	0.00%	0.00%	0.00%

Income Taxes - Net Deferred Tax

Income Taxes - Net Deferred Tax Assets (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 48,496	$ 36,880
Research and development tax credit carryforwards	7,725	7,078
Accruals and other temporary differences	3,578	4,897
Debt discount	(14,630)	0
Capitalized research and development expenses, net	29,673	34,579
Total deferred tax assets	74,842	83,434
Valuation allowance	(74,842)	(83,434)	$ (54,773)	$ (33,826)
Net deferred tax asset	$ 0	$ 0

Income Taxes - Changes in Valua

Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes Disclosure [Line Items]
Valuation allowance as of beginning of year	$ (83,434)	$ (54,773)	$ (33,826)
Valuation allowance as of end of year	(74,842)	(83,434)	(54,773)
Decreases Recorded as Benefit to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increases) recorded as benefit to income tax provision	36,606	4,771	2,858
Decreases Recorded as Benefit to Equity [Member]
Income Taxes Disclosure [Line Items]
Decreases (increases) recorded as benefit to income tax provision	24,537
Increases Recorded to Income Tax Provision [Member]
Income Taxes Disclosure [Line Items]
Decreases (increases) recorded as benefit to income tax provision	$ (52,551)	$ (33,432)	$ (23,805)

Quarterly Financial Data (una73

Quarterly Financial Data (unaudited) - Recurring Adjustments Necessary for Fair Statement (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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]
Revenues	$ 355	$ 0	$ 0	$ 0					$ 355
Operating expenses	48,132	31,221	26,902	23,782	$ 21,552	$ 17,435	$ 14,120	$ 16,673
Net loss	$ (50,534)	$ (34,188)	$ (28,880)	$ (23,879)	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (137,481)	$ (71,894)	$ (46,315)
Net loss per common share—basic and diluted	$ (1.38)	$ (1.07)	$ (0.91)	$ (0.75)	$ (0.79)	$ (0.65)	$ (0.63)	$ (0.78)	$ (4.16)	$ (2.84)	$ (2.15)
Weighted average common shares—basic and diluted	36,644	31,931	31,826	31,704	29,347	27,524	22,666	21,570	33,027	25,297	21,497