Enanta Pharmaceuticals (ENTA) 10-K 9 Dec 16 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Sep. 30, 2016	Dec. 01, 2016	Mar. 31, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	ENTA
Entity Registrant Name	ENANTA PHARMACEUTICALS INC
Entity Central Index Key	1,177,648
Current Fiscal Year End Date	--09-30
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		19,038,927
Entity Public Float			$ 556,755,420

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Current assets:
Cash and cash equivalents	$ 16,577	$ 21,726
Short-term marketable securities	193,507	123,479
Accounts receivable	12,841	15,289
Unbilled receivables		433
Deferred tax assets		1,447
Prepaid expenses and other current assets	9,231	8,267
Total current assets	232,156	170,641
Property and equipment, net	8,004	5,886
Long-term marketable securities	32,119	64,238
Deferred tax assets	8,390	4,640
Restricted cash	608	608
Total assets	281,277	246,013
Current liabilities:
Accounts payable	3,377	1,543
Accrued expenses and other current liabilities	4,512	3,962
Income taxes payable		1,199
Total current liabilities	7,889	6,704
Warrant liability	1,251	1,276
Series 1 nonconvertible preferred stock	159	163
Other long-term liabilities	2,042	1,713
Total liabilities	11,341	9,856
Commitments and contingencies (Note 12)
Stockholders' equity:
Common stock; $0.01 par value, 100,000,000 shares authorized; 19,035,910 and 18,716,834 shares issued and outstanding at September 30, 2016 and September 30, 2015, respectively	190	187
Additional paid-in capital	242,081	229,957
Accumulated other comprehensive income	19	33
Retained earnings	27,646	5,980
Total stockholders' equity	269,936	236,157
Total liabilities and stockholders' equity	$ 281,277	$ 246,013

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2016	Sep. 30, 2015
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	19,035,910	18,716,834
Common stock, shares outstanding	19,035,910	18,716,834

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Revenue
Royalties	$ 57,692	$ 34,077
Milestones	30,000	125,000	$ 40,000
Other	576	1,803	7,741
Total revenue	88,268	160,880	47,741
Operating expenses:
Research and development	40,461	23,189	18,740
General and administrative	16,966	13,543	10,016
Total operating expenses	57,427	36,732	28,756
Income from operations	30,841	124,148	18,985
Other income (expense):
Interest income	1,735	968	467
Interest expense	(45)	(8)	(18)
Change in fair value of warrant liability and Series 1 nonconvertible preferred stock, net	29	347	(166)
Total other income (expense), net	1,719	1,307	283
Income before income taxes	32,560	125,455	19,268
Income tax (expense) benefit	(10,894)	(46,463)	15,170
Net income	$ 21,666	$ 78,992	$ 34,438
Net income per common share:
Basic	$ 1.14	$ 4.23	$ 1.88
Diluted	$ 1.13	$ 4.09	$ 1.80
Weighted average common shares outstanding:
Basic	18,929	18,673	18,355
Diluted	19,224	19,295	19,185

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 21,666	$ 78,992	$ 34,438
Other comprehensive income (loss):
Net unrealized gains (losses) on marketable securities, net of tax of ($9), $20, and $0	(14)	133	(98)
Total other comprehensive income (loss)	(14)	133	(98)
Comprehensive income	$ 21,652	$ 79,125	$ 34,340

Consolidated Statements of Com6

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net unrealized gains on marketable securities, tax	$ (9)	$ 20	$ 0

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Beginning Balance at Sep. 30, 2013	$ 110,468	$ 181	$ 217,741	$ (2)	$ (2)	$ (107,450)
Beginning Balance, Shares at Sep. 30, 2013		18,138,597		(208,816)
Exercise of stock options	1,054	$ 7	1,047
Exercise of stock options, Shares		664,793
Stock-based compensation expense	2,682		2,682
Income tax benefit from stock option exercises	110		110
Other comprehensive income (loss)	(98)				(98)
Net income	34,438					34,438
Ending Balance at Sep. 30, 2014	148,654	$ 188	221,580	$ (2)	(100)	(73,012)
Ending Balance, Shares at Sep. 30, 2014		18,803,390		(208,816)
Exercise of stock options	726	$ 1	725
Exercise of stock options, Shares		122,260
Stock-based compensation expense	5,838		5,838
Income tax benefit from stock option exercises	1,814		1,814
Other comprehensive income (loss)	$ 133				133
Retirement of treasury stock		$ (2)		$ 2
Retirement of treasury stock, Shares	208,816,000	(208,816)		208,816
Net income	$ 78,992					78,992
Ending Balance at Sep. 30, 2015	236,157	$ 187	229,957		33	5,980
Ending Balance, Shares at Sep. 30, 2015		18,716,834
Exercise of stock options	$ 1,026	$ 3	1,023
Exercise of stock options, Shares	319,076	319,076
Stock-based compensation expense	$ 9,354		9,354
Income tax benefit from stock option exercises	1,747		1,747
Other comprehensive income (loss)	(14)				(14)
Net income	21,666					21,666
Ending Balance at Sep. 30, 2016	$ 269,936	$ 190	$ 242,081		$ 19	$ 27,646
Ending Balance, Shares at Sep. 30, 2016		19,035,910

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities
Net income	$ 21,666	$ 78,992	$ 34,438
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	9,354	5,838	2,682
Depreciation and amortization expense	1,661	639	352
Change in fair value of warrant liability and Series 1 nonconvertible preferred stock	(29)	(347)	166
Premium on marketable securities	(518)	(3,042)	(2,002)
Amortization of premium on marketable securities	1,511	2,244	2,120
Deferred income taxes	(2,294)	11,028	(15,211)
Income tax benefit from the exercise of stock options	(1,747)	(1,814)	(110)
Other non-cash items	34	(35)	2
Change in operating assets and liabilities:
Accounts receivable	2,448	(13,565)	(916)
Unbilled receivables	433	2,337	(1,986)
Prepaid expenses and other current assets	(964)	(6,680)	568
Accounts payable	1,451	(543)	399
Accrued expenses	1,858	(227)	(183)
Income taxes payable	548	1,199
Other long-term liabilities	397	649	(140)
Net cash provided by operating activities	35,809	76,673	20,179
Cash flows from investing activities
Purchases of property and equipment	(4,738)	(2,336)	(1,028)
Purchases of marketable securities	(192,429)	(196,304)	(98,405)
Sale of marketable securities		2,210	8,421
Maturities of marketable securities	153,504	108,407	91,506
Other investing activities		(163)	3
Net cash provided by (used in) investing activities	(43,663)	(88,186)	497
Cash flows from financing activities
Proceeds from exercise of stock options	1,026	726	1,054
Payments of capital lease obligations	(68)
Income tax benefit from the exercise of stock options	1,747	1,814	110
Net cash provided by financing activities	2,705	2,540	1,164
Net increase (decrease) in cash and cash equivalents	(5,149)	(8,973)	21,840
Cash and cash equivalents at beginning of period	21,726	30,699	8,859
Cash and cash equivalents at end of period	16,577	21,726	30,699
Supplemental disclosure of cash flow information:
Cash paid for income taxes	12,616	40,072	148
Supplemental disclosure of non-cash financing activities:
Exercise of Series 1 warrants into Series 1 nonconvertible preferred stock			$ 206
Non-cash items:
Fixed assets financed through capital lease		598
Fixed assets financed by landlord		239
Purchases of fixed assets included in accounts payable and accrued expenses	$ 637	$ 1,562

Nature of the Business

Nature of the Business	12 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business	1. Nature of the Business Enanta Pharmaceuticals, Inc. (the “Company”), incorporated in Delaware in 1995, is a research and development-focused biotechnology company that uses its robust chemistry-driven approach and drug discovery capabilities to create small molecule drugs primarily for the treatment of viral infections and liver diseases. The Company’s research and development is currently focused on four disease targets: hepatitis C virus (“HCV”); hepatitis B virus (“HBV”); non-alcoholic steatohepatitis (“NASH”); and respiratory syncytial virus (“RSV”). The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, the uncertainties of research and development, competition from technological innovations of others, dependence on collaborative arrangements, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure, and extensive compliance reporting capabilities.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements include those of the Company and its subsidiary, Enanta Pharmaceuticals Security Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, management’s judgments of separate units of accounting and best estimate of selling price of those units of accounting within its revenue arrangements; valuation of warrants, Series 1 nonconvertible preferred stock and stock-based awards; and the accounting for income taxes, including uncertain tax positions and the valuation of net deferred tax assets. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates. Cash Equivalents and Marketable Securities The Company considers all short-term, highly liquid investments with original maturities of ninety days or less at acquisition date to be cash equivalents. Marketable securities with original maturities of greater than 90 days and remaining maturities of less than one year from the balance sheet date are classified as short-term marketable securities. Marketable securities with remaining maturities of greater than one year from the balance sheet date are classified as long-term marketable securities. The Company classifies all of its marketable securities as available-for-sale. All marketable securities are held with one investment manager. The Company continually evaluates the credit ratings of its investment portfolio and underlying securities. The Company invests in accordance with its investment policy and invests at the date of purchase in securities with a rating of A3 or higher and A—or higher according to Moody’s and S&P, respectively. The Company reports available-for-sale investments at fair value as of each balance sheet date and records any unrealized gains or losses as a component of stockholders’ equity. The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense), net within the consolidated statements of operations. If any adjustment to fair value reflects a decline in the value of the investment, the Company considers available evidence to evaluate the extent to which the decline is “other than temporary” and reduces the investment to fair value through a charge to the consolidated statements of operations. There were no such adjustments necessary during the years ended September 30, 2016, 2015 and 2014. Restricted Cash As of September 30, 2016 and 2015, the Company had an outstanding letter of credit collateralized by a money market account of $608 to the benefit of the landlord of the Company’s current building lease. This amount was classified as long-term restricted cash as of September 30, 2016 and 2015. Concentration of Credit Risk and of Significant Customers and Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities, accounts receivable and unbilled receivables. The Company has all cash and investment balances at one accredited financial institution, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company has historically generated all of its revenue from its collaborative research and license agreements as well as a U.S. government contract (see Note 7). As of September 30, 2016, accounts receivable consisted of amounts due from the Company’s principal collaborator (see Note 7). The Company is completely dependent on third-party manufacturers for product supply for preclinical and clinical research activities in its non-partnered programs. The Company relies and expects to continue to rely exclusively on several manufacturers to supply it with its requirements for the active pharmaceutical ingredients related to these programs. These research programs would be adversely affected by a significant interruption in the supply of its active pharmaceutical ingredients. Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. A fair value hierarchy is based on three levels of inputs which are used to measure fair value, of which the first two levels are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices) such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s instruments that are carried at fair value are cash equivalents, marketable securities and the warrant and Series 1 nonconvertible preferred stock liabilities. The carrying values of accounts receivable and unbilled receivables, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these assets and liabilities. Property and Equipment Property and equipment are stated at cost less accumulated depreciation or amortization. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Laboratory and office equipment 5 years Leasehold improvements Shorter of life of lease or estimated useful life Purchased software 3 years Computer equipment 3 years Furniture 7 years Expenditures for repairs and maintenance of assets are charged to expense as incurred. Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Upon retirement or sale, the cost and related accumulated depreciation or amortization of assets disposed are removed from the accounts and any resulting gain or loss is included in income from operations in the consolidated statements of operations. Impairment of Long-Lived Assets The Company reviews 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. To date, the Company has not recorded any impairment losses on long-lived assets. Revenue Recognition The Company’s revenue has been generated primarily through collaborative research and license agreements. The terms of these agreements contain multiple deliverables, which may include (i) licenses, (ii) research and development activities, and (iii) participation in joint research and development steering committees. The terms of these agreements may include nonrefundable upfront license fees, payments for research and development activities, payments based upon the achievement of certain milestones, and royalty payments based on product sales derived from the collaboration. In all instances, revenue is recognized only when the price is fixed or determinable, persuasive evidence of an arrangement exists, delivery has occurred or the services have been rendered, collectibility of the resulting receivable is reasonably assured, and the Company has fulfilled its performance obligations under the contract. On October 1, 2011, the Company adopted Accounting Standards Update No. 2009-13, Multiple-Deliverable Revenue Arrangements In determining the separate units of accounting, the Company evaluates whether the license has standalone value to the collaborator based on consideration of the relevant facts and circumstances for each arrangement. Factors considered in this determination include the research and development capabilities of the collaborator and the availability of relevant research expertise in the marketplace. In addition, the Company considers whether or not (i) the collaborator can use the license for its intended purpose without the receipt of the remaining deliverables, (ii) the value of the license is dependent on the undelivered items, and (iii) the collaborator or other vendors can provide the undelivered items. For all periods presented, whenever the Company determines that an element is delivered over a period of time, revenue is recognized using either a proportional performance model or a straight-line model over the period of performance, which is typically the research and development term. Full-time equivalents (“FTEs”) are typically used as the measure of performance. At each reporting period, the Company reassesses its cumulative measure of performance and makes appropriate adjustments, if necessary. The Company recognizes revenue using the proportional performance model whenever the Company can make reasonably reliable estimates of the level of effort required to complete its performance obligations under an arrangement. Revenue recognized under the proportional performance model at each reporting period is determined by multiplying the total expected payments under the contract (excluding royalties and payments contingent upon achievement of milestones) by the ratio of the level of effort incurred to date to the estimated total level of effort required to complete the performance obligations under the arrangement. Revenue is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the proportional performance model as of each reporting period. Alternatively, if the Company cannot make reasonably reliable estimates of the level of effort required to complete its performance obligations under an arrangement, then revenue under the arrangement is recognized on a straight-line basis over the period expected to complete the Company’s performance obligations. If and when a contingent milestone payment is earned, the additional consideration to be received is allocated to the separate units of accounting in the arrangement based on their relative selling prices at the inception of the arrangement. Revenue is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined on a straight-line basis as of the period end date. If the Company cannot reasonably estimate when its performance obligation period ends, then revenue is deferred until the Company can reasonably estimate when the performance obligation period ends. Royalty revenue is recognized based on contractual terms when reported sales are reliably measurable and collectibility is reasonably assured, provided that there are no performance obligations remaining. During the years ended September 30, 2016, 2015 and 2014, the Company also generated revenue from a government contract, under which the Company was reimbursed for certain allowable costs for the funded project. Revenue from the government contract was recognized when the related service was performed. The related costs incurred by the Company under the government contract were included in research and development expenses in the consolidated statements of operations. Amounts received prior to satisfying all revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the next twelve months of the consolidated balance sheet date are classified as long-term deferred revenue. In the event that a collaborative research and license agreement is terminated and the Company then has no further performance obligations, the Company recognizes as revenue any amounts that had not previously been recorded as revenue but were classified as deferred revenue at the date of such termination. Research and Development Costs Research and development costs are expensed as incurred. Included in research and development costs are wages, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including facility-related expenses and external costs of outside contractors engaged to conduct both preclinical and clinical studies and manufacture quantities of product for preclinical and clinical studies. The Company also includes in research and development expense the costs to complete the Company’s obligations under research collaborations. 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. Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees at fair value on the date of grant. The Company uses the Black-Scholes option-pricing model in the valuation of its stock options. The fair value of performance-based awards is based on intrinsic value of the stock on the date of grant. The Company uses the Monte-Carlo simulation in order to calculate the fair value of the market-based awards. The fair value of options is recognized as stock-based compensation 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-based and market-based conditions. The Company records stock-based compensation expense related to performance-based awards when the performance-based targets are probable of being achieved. 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. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to be recognized in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. As discussed in Note 13, the Company adopted ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes Net Income per Share Basic net income per common share is computed by dividing the net income by the weighted average number of shares of common stock outstanding for the period. Diluted net income per common share is computed by dividing net income by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options, unvested restricted stock units and outstanding warrants. Market-based awards are included in diluted earnings per common share to the extent they would have vested if the period end date was the market criteria measurement date. Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is a biotechnology company focused on discovering and developing small molecule drugs for the treatment of viral infections and liver diseases. Revenue is generated exclusively from transactions occurring with partners located in the United States and all assets are held in the United States. Comprehensive Income Comprehensive income includes net income as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income is unrealized gains and losses on available-for-sale marketable securities. Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers , Revenue from Contract with Customers: Principal versus Agent Considerations Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

Fair Value of Financial Assets

Fair Value of Financial Assets and Liabilities	12 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Fair Value of Financial Assets and Liabilities	3. Fair Value of Financial Assets and Liabilities The following tables present information about the Company’s financial assets and liabilities that were subject to fair value measurement on a recurring basis as of September 30, 2016 and 2015 and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value: Fair Value Measurements at September 30, 2016 Using: Level 1 Level 2 Level 3 Total Assets: U.S. Treasury notes $ 69,608 $ — $ — $ 69,608 Cash equivalents 15,295 — — 15,295 Corporate bonds — 76,073 — 76,073 Commercial paper — 49,900 — 49,900 U.S. Agency bonds — 30,045 — 30,045 $ 84,903 $ 156,018 $ — $ 240,921 Liabilities: Warrant liability $ — $ — $ 1,251 $ 1,251 Series 1 nonconvertible preferred stock — — 159 159 $ — $ — $ 1,410 $ 1,410 Fair Value Measurements at September 30, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 21,327 $ — $ — $ 21,327 Corporate bonds — 151,020 — 151,020 U.S. Agency bonds — 36,697 — 36,697 $ 21,327 $ 187,717 $ — $ 209,044 Liabilities: Warrant liability $ — $ — $ 1,276 $ 1,276 Series 1 nonconvertible preferred stock — — 163 163 $ — $ — $ 1,439 $ 1,439 Cash equivalents at September 30, 2016 and 2015 consist of money market funds. During the years ended September 30, 2016, 2015, and 2014, there were no transfers between Level 1, Level 2 and Level 3. As of September 30, 2016 and 2015, the warrant liability was comprised of the value of the warrants for the purchase of Series 1 nonconvertible preferred stock measured at fair value. As of September 30, 2016 and 2015, the outstanding Series 1 nonconvertible preferred stock was also measured at fair value. The fair values of these instruments were based on significant inputs not observable in the market, which represented a Level 3 measurement within the fair value hierarchy. The Company utilized a probability-weighted valuation model which takes into consideration various outcomes that may require the Company to transfer assets upon exercise. Changes in the fair values of the warrant liability and Series 1 nonconvertible preferred stock are recognized in other income (expense), net, in the consolidated statements of operations. The recurring Level 3 fair value measurements of the Company’s warrant liability and Series 1 nonconvertible preferred stock using probability-weighted discounted cash flow include the following significant unobservable inputs: Range (Weighted Average) Unobservable Input 2016 2015 Warrant liability and Series 1 nonconvertible preferred stock Probabilities of payout Periods in which payout is expected to occur Discount rate 0% - 60% 2017 - 2018 4.50% 5% - 60% 2016 - 2017 4.25% The following table provides a rollforward of the aggregate fair values of the Company’s warrants for the purchase of Series 1 nonconvertible preferred stock and the outstanding Series 1 nonconvertible preferred stock for which fair value is determined by Level 3 inputs: Warrant Series 1 Balance, September 30, 2013 $ 1,620 $ — Warrants exercised (206 ) — Series 1 nonconvertible preferred stock issued — 206 Increase (decrease) in fair value 170 (4 ) Balance, September 30, 2014 $ 1,584 $ 202 Decrease in fair value (308 ) (39 ) Balance, September 30, 2015 $ 1,276 $ 163 Decrease in fair value (25 ) (4 ) Balance, September 30, 2016 $ 1,251 $ 159

Marketable Securities

Marketable Securities	12 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities As of September 30, 2016 and 2015, the fair value of available-for-sale marketable securities, by type of security, was as follows: September 30, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate bonds $ 76,077 $ 27 $ (31 ) $ 76,073 U.S. Treasury notes 69,579 38 (9 ) 69,608 Commercial paper 49,900 — — 49,900 U.S. Agency bonds 30,040 15 (10 ) 30,045 $ 225,596 $ 80 $ (50 ) $ 225,626 September 30, 2015 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate bonds $ 151,012 $ 77 $ (69 ) $ 151,020 U.S. Agency bonds 36,652 45 — 36,697 $ 187,664 $ 122 $ (69 ) $ 187,717 As of September 30, 2016 and 2015, marketable securities consisted of investments that mature within one year, with the exception of certain corporate bonds, U.S. Treasury notes and U.S. Agency bonds, which have maturities between one and three years and an aggregate fair value of $32,119 and $64,238, respectively.

Property and Equipment

Property and Equipment	12 Months Ended
	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Property and Equipment	5. Property and Equipment Property and equipment consisted of the following as of September 30, 2016 and 2015: September 30, 2016 2015 Laboratory and office equipment $ 7,440 $ 5,907 Leasehold improvements 3,715 1,290 Purchased software 705 443 Furniture 551 496 Construction in progress 504 1,889 Computer equipment 187 137 13,102 10,162 Less: Accumulated depreciation and amortization (5,098 ) (4,276 ) $ 8,004 $ 5,886 Depreciation and amortization expense for property and equipment, including assets acquired under capital leases, was $1,661, $639 and $352 for the years ended September 30, 2016, 2015, and 2014, respectively.

Accrued Expenses, Other Current

Accrued Expenses, Other Current Liabilities and Other Long-Term Liabilities	12 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Accrued Expenses, Other Current Liabilities and Other Long-Term Liabilities	6. Accrued Expenses, Other Current Liabilities and Other Long-Term Liabilities Accrued expenses, other current liabilities and other long-term liabilities consisted of the following as of September 30, 2016 and 2015: September 30, 2016 2015 Accrued expenses and other current liabilities: Accrued payroll and related expenses $ 2,384 $ 1,622 Accrued preclinical and clinical expenses 899 237 Accrued vendor manufacturing 441 18 Accrued professional fees 393 338 Capital lease obligation 73 69 Accrued fixed assets 2 1,307 Accrued other 320 371 $ 4,512 $ 3,962 Other long-term liabilities: Uncertain tax positions $ 745 $ 448 Accrued rent expense 696 628 Capital lease obligation 458 529 Asset retirement obligation 143 108 $ 2,042 $ 1,713

Collaboration Agreements

Collaboration Agreements	12 Months Ended
	Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Collaboration Agreements	7. Collaboration Agreements AbbVie Collaboration On November 27, 2006, the Company entered into a Collaborative Development and License Agreement (the “AbbVie Agreement”) with Abbott Laboratories to identify, develop and commercialize HCV NS3 and NS3/4A protease inhibitor compounds, including paritaprevir and glecaprevir (ABT-493). The agreement was assigned by Abbott to AbbVie Inc. on January 1, 2013 in connection with Abbott’s transfer of its research-based pharmaceuticals business to AbbVie. Under the terms of the AbbVie Agreement, as amended, AbbVie paid the Company upfront license payments and FTE reimbursements to fund research activities. The Company is also eligible to receive milestone payments for the successful development by AbbVie of one or more HCV compounds, as well as annually tiered, per-product royalties on the portion of AbbVie’s net sales of its HCV treatment regimens allocated to the protease inhibitor product. The Company determined that the deliverables under the AbbVie Agreement included (i) the non-exclusive, royalty-free, worldwide research license and the exclusive, royalty-bearing development and commercialization license, (ii) the research services, and (iii) a commitment to participate on a steering committee, all of which were to be delivered over a three-year period. The Company concluded that the license did not have standalone value as it was dependent, in part, upon the Company’s continuing involvement in the HCV protease inhibitor research and its involvement in the joint steering committee. Additionally, the undelivered items, including the Company’s participation in the joint steering committee, which was considered participatory due to its decision making responsibilities, and the research services, did not have VSOE or VOE of fair value. Therefore, the license, the research services, and the joint steering committee participation were treated as a single unit of accounting. Accordingly, all amounts received were deferred, and revenue was recognized using the proportional performance model over the period during which the Company performed research services in connection with the AbbVie Agreement, as amended. Subsequent to the research and evaluation period, which ended in June 2011, all decisions related to the development, commercialization and marketing have been made by AbbVie. The Company has the right to continue to attend the joint steering committee meetings to monitor the development and marketing plans; however, the Company has no decision-making rights. As such, the joint steering committee commitment became protective in nature as of June 16, 2011. During the years ended September 30, 2016, 2015, and 2014, the Company received $30,000, $125,000 and $40,000, respectively, in milestone payments under the AbbVie Agreement as a result of AbbVie’s commercialization and regulatory approvals and regulatory filings for a regimen that included paritaprevir. From commencement of the collaboration through September 30, 2016, the Company received a total of $394,000 under the AbbVie Agreement consisting of an upfront license payment, research funding, milestone payments, royalties and preferred stock financing. As of September 30, 2016, the Company is eligible to receive additional milestone payments of up to $80,000 upon AbbVie’s achievement of similar commercialization regulatory approval milestones in the U.S. and other major world markets for commercialization regulatory approvals of glecaprevir. The Company is also eligible to receive annually tiered, per-product royalties ranging from the low double digits up to twenty percent, or on a blended basis from the low double digits up to the high teens, on the portion of AbbVie’s net sales of its HCV treatment regimens allocated to glecaprevir. Under the terms of the agreement, as amended in October 2014, 30% of net sales of each 3-DAA regimen containing paritaprevir, and 45% of net sales of each 2-DAA regimen containing paritaprevir, will be allocated to paritaprevir. In the case of glecaprevir, 50% of AbbVie’s net sales of this 2-DAA regimen would be allocated to glecaprevir. The Company began earning and receiving royalty payments on net sales of regimens containing paritaprevir during the year ended September 30, 2015. Since the Company completed all its performance obligations under the AbbVie Agreement by the end of fiscal 2011, any milestone payments received since then have been and will be recognized as revenue when achieved by AbbVie. Royalties owed to the Company under the agreement can be reduced by AbbVie in certain circumstances, including (i) if AbbVie exercises its right to license or otherwise acquire rights to intellectual property controlled by a third party where a product could not be legally developed or commercialized in a country without the third-party intellectual property right, (ii) where a product developed under the collaboration agreement is sold in a country and not covered by a valid patent claim in such country, and (iii) where sales of a generic product are equal to at least a specified percentage of AbbVie’s market share of a product in a country. AbbVie’s obligation to pay royalties on a product developed under the agreement expires on a country-by-country basis upon the later of (i) the date of expiration of the last of the licensed patents with a valid claim covering the product in the applicable country, or (ii) ten years after the first commercial sale of the product in the applicable country. Subject to certain exceptions, a party’s rights and obligations under the agreement continue until (i) such time as AbbVie is no longer developing a product candidate or (ii) if, as of the time AbbVie is no longer developing any product candidates, AbbVie is commercializing any other protease inhibitor product, such time as all royalty terms for all covered products have ended. Accordingly, the final expiration date of the agreement is currently indeterminable. Either party may terminate the agreement for cause in the event of a material breach, subject to prior notice and the opportunity to cure, or in the event of the other party’s bankruptcy. Additionally, AbbVie may terminate the agreement for any reason upon specified prior notice. If the Company terminates the agreement for cause or AbbVie terminates without cause, any licenses and other rights granted to AbbVie will terminate and AbbVie will be deemed to have granted the Company (i) a non-exclusive, perpetual, fully-paid, worldwide, royalty-free license, with the right to sublicense, under AbbVie’s intellectual property used in any product candidate, and (ii) an exclusive (even as to AbbVie), perpetual, fully-paid, worldwide, royalty-free license, with the right to sublicense, under AbbVie’s interest in any joint intellectual property rights to develop product candidates resulting from covered compounds and to commercialize any products derived from such compounds. Upon the Company’s request, AbbVie will also transfer to the Company all right, title and interest in any related product trademarks, regulatory filings and clinical trials. If AbbVie terminates the agreement for the Company’s uncured breach, the milestone and royalty payments payable by AbbVie may be reduced, the licenses granted to AbbVie will remain in place, the Company will be deemed to have granted AbbVie an exclusive license under the Company’s interest in joint intellectual property, AbbVie will continue to have the right to commercialize any covered products, and all rights and licenses granted to the Company by AbbVie will terminate. NIAID Contract On September 30, 2011, the Company entered into a contract with the National Institute of Allergy and Infectious Diseases (“NIAID”), a division of the National Institutes of Health (“NIH”), providing development funding to the Company for the preclinical and clinical development of a bridged bicyclic antibiotic. The contract was completed in August 2015 upon the Company’s delivery of the study report for the Phase 1 clinical study. The Company recognized revenue under this contract as development services were performed in accordance with the funding agreement. During the years ended September 30, 2016, 2015, and 2014, the Company recognized revenue of $576, $1,803 and $7,741, respectively, under this contract. The Company received aggregate payments of $20,637 under the NIAID contract from its commencement through January 31, 2016.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Stockholders' Equity	8. Stockholders’ Equity On March 26, 2013, the Company completed an initial public offering of its common stock, which resulted in the sale of 4,600,000 shares. The Company received net proceeds from the IPO of $59,892 based upon the price of $14.00 per share and after deducting underwriting discounts and commissions paid by the Company. Upon the closing of the initial public offering, all outstanding shares of the Company’s redeemable convertible preferred stock and convertible preferred stock were converted into 11,656,875 shares of common stock. Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to receive such dividends as may be declared by the board of directors, if any. The Company retired all 208,816 shares of treasury stock effective September 30, 2015, which resulted in a decrease in the same number of issued shares of common stock.

Series 1 Nonconvertible Preferr

Series 1 Nonconvertible Preferred Stock and Warrants	12 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Series 1 Nonconvertible Preferred Stock and Warrants	9. Series 1 Nonconvertible Preferred Stock and Warrants The Company’s Certificate of Incorporation authorizes the issuance of up to 1,999,989 shares of Series 1 nonconvertible preferred stock at a par value of $0.01 per share. Holders of Series 1 nonconvertible preferred stock are not entitled to receive dividends. In the event of any liquidation, deemed liquidation, dissolution or winding up of the Company, the Series 1 nonconvertible preferred stockholders are entitled to receive in preference to all other stockholders, an amount equal to $1.00 per share, adjusted for any stock dividends, stock splits or reclassifications. Series 1 nonconvertible preferred stockholders will not be entitled to vote unless required by the Company pursuant to the laws of the State of Delaware. The Company may redeem the Series 1 nonconvertible preferred stock with the approval of the holders of a majority of the outstanding shares of Series 1 nonconvertible preferred stock at a redemption price of $1.00 per share. The Company must redeem the stock within 60 days of such election. Shares that are redeemed will be retired or canceled and not reissued by the Company. In October and November 2010, a total of 1,999,989 warrants to purchase Series 1 nonconvertible preferred stock were issued. These warrants expire on October 4, 2017. As these warrants are free-standing financial instruments that may require the Company to transfer assets upon exercise, up to a maximum of $2,000, these warrants are classified as liabilities on the Company’s consolidated balance sheet. The following table summarizes the Company’s outstanding warrants: Series 1 Nonconvertible Preferred Stock Warrants Exercisable Weighted Average Exercise Price Outstanding, as of September 30, 2013 1,999,989 $ 0.01 Granted — — Expired — — Exercised (225,408 ) $ 0.01 Outstanding, as of September 30, 2014 1,774,581 $ 0.01 Granted — — Expired — — Exercised — — Outstanding, as of September 30, 2015 1,774,581 $ 0.01 Granted — — Expired — — Exercised — — Outstanding, as of September 30, 2016 1,774,581 $ 0.01 For the years ended September 30, 2016, 2015, and 2014, the remeasurement of the warrants and Series 1 nonconvertible preferred stock resulted in income (expense) recorded in the consolidated statements of operations of $29, $347, and $(166), respectively. As of September 30, 2016 and 2015, the total fair value of the Series 1 nonconvertible preferred stock was $159 and $163, respectively. As of September 30, 2016 and 2015, the total fair value of the Series 1 nonconvertible preferred stock warrants was $1,251 and $1,276, respectively.

Stock-Based Awards

Stock-Based Awards	12 Months Ended
	Sep. 30, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Awards	10. Stock-Based Awards The Company’s 2012 Equity Incentive Plan (the “2012 Plan”) permits the Company to sell or issue awards of common stock or restricted common stock or to grant awards of incentive stock options or nonqualified stock options for the purchase of common stock, restricted stock units, performance units, stock appreciation rights or other cash incentive awards, to employees, members of the board of directors and consultants of the Company. The number of shares of common stock that may be issued under the 2012 Plan is subject to increase by the number of shares forfeited under any options terminated and not exercised under the 2012 Plan or a predecessor plan, known as the 1995 Equity Incentive Plan, as well as by the number of shares added on the first day of each fiscal year, which is the lowest amount among the following: (i) 3% of the Company’s outstanding shares of common stock as of that date, (ii) 2,088,167 shares of common stock, or (iii) an amount determined by the Compensation Committee of the Board of Directors. On October 1, 2015, the number of shares of common stock that might be issued under the 2012 Plan was increased by 561,505 shares. As of September 30, 2016, 487,906 shares remained available for future award. The 2012 Plan replaces and is the successor to the 1995 Equity Incentive Plan (the “1995 Plan”). The 1995 Plan provided for the Company to sell or issue awards of common stock or restricted common stock, or to grant awards of incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. Sales, issuances or grants of shares entitle the holder to purchase common stock from the Company, for a specified exercise price, during a period specified by the applicable equity award agreement. Upon the closing of the Company’s initial public offering, all remaining shares reserved for issuance under the 1995 Plan were transferred to the 2012 Plan and no further awards were or will be made under the 1995 Plan. Under the Company’s Employee Stock Purchase Plan (“ESPP”) a total of 185,614 shares of common stock are reserved for issuance. As of September 30, 2016, the Company had not commenced any offering under the ESPP and no plan shares have been issued. The Company applies the fair value recognition provisions for all stock-based awards granted or modified. In the case of service-based awards, the compensation cost is recorded over the requisite service period of the award on the straight-line method based on the grant-date fair value. The requisite service period for service-based option awards is generally four years. Options granted under the 2012 Plan to employees generally vest between one and four years and expire after ten years. Stock Option Valuation The fair value of each stock option award is estimated on the date of grant using the Black-Scholes option-pricing model. The Company estimates expected volatility based on the historical volatility of publicly traded peer companies. The Company expects to continue to do so until such time as adequate historical data regarding the volatility of the Company’s traded stock price following our March 2013 IPO is available. The expected term of the Company’s options has been determined utilizing the “simplified” method for awards that qualify as “plain-vanilla” options. The Company expects to continue to utilize this method until such time as we have adequate historical data regarding our employee exercise patterns. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. The expected dividend yield is 0 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 awards are as follows, presented on a weighted average basis: Years Ended September 30, 2016 2015 2014 Risk-free interest rate 1.77 % 1.85 % 1.89 % Expected term (in years) 6.10 6.03 6.06 Expected volatility 70 % 74 % 75 % Expected dividends 0 % 0 % 0 % The Company recognizes stock-based compensation expense for only the portion of awards that are expected to vest. In developing a forfeiture rate estimate, the Company has considered its historical experience to estimate pre-vesting forfeitures for service-based awards. The impact of a forfeiture rate adjustment will be recognized in full in the period of adjustment, and if the actual forfeiture rate is materially different from the Company’s estimate, the Company may be required to record adjustments to its stock-based compensation expense in future periods. As required by the 1995 Plan and the 2012 Plan, the exercise price for awards granted is not to be less than the fair value of common shares as estimated by the Company as of the date of grant. The following table summarizes stock option activity, including aggregate intrinsic value for the year ended September 30, 2016: Shares Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term in years Aggregate Intrinsic Value Outstanding as of September 30, 2015 1,752,010 $ 23.76 7.2 $ 25,093 Granted 509,680 30.31 Exercised (319,076 ) 3.22 Forfeited (47,158 ) 33.04 Outstanding, September 30, 2016 1,895,456 $ 28.75 7.6 $ 7,369 Options vested and expected to vest as of September 30, 2016 1,772,221 $ 29.65 7.6 $ 5,958 Options exercisable as of September 30, 2016 957,141 $ 25.52 6.9 $ 5,727 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. The following table summarizes additional exercise and grant date information: Years Ended September 30, 2016 2015 2014 Aggregate intrinsic value of stock options exercised $ 7,705 $ 4,827 $ 3,219 Proceeds from stock options exercised $ 1,026 $ 726 $ 1,054 Weighted average grant date fair value of options granted (per share) $ 19.12 $ 27.77 $ 19.98 Performance-Based Options In March 2013, the Company granted to certain executives 167,052 options that vest upon the achievement of certain performance-based targets. The aggregate grant date fair value of these options was $2,479. During the years ended September 30, 2016 and 2014, certain performance-based targets were achieved and the Company recorded stock-based compensation expense of $620 and $206, respectively, related to achievement of those targets. No stock-based compensation expense related to these options was recognized during the year ended September 30, 2015, as none of the performance-based targets was deemed to have become probable of being achieved in that period. Market and Performance-Based Stock Unit Awards The Company awards both performance share units, or PSUs, and relative total shareholder return units, or rTSRUs, to its executive officers. The number of units represents the target number of shares of common stock that may be earned; however, the actual number of shares that may be earned ranges from 0% to 200% of the target number. The PSUs will vest and result in issuance, or settlement, of common shares for each recipient, based upon the recipient’s continued employment with the Company and the Company’s achievement of specified research and development milestones. The requisite service period of the PSUs is generally 2 years. During the years ended September 30, 2016 and 2015, the Company recorded no compensation expense related to the PSU awards as none of the performance-based targets was probable of being achieved. There were no performance share units granted during the year ended September 30, 2014. The rTSRUs will vest and result in the issuance of common stock based upon the recipient’s continuing employment with the Company and the relative ranking of the total shareholder return, or TSR, of the Company’s common stock in relation to the TSR of the component companies in the NASDAQ Biotech Index over a two-year period based on a comparison of average closing stock prices in specified periods noted in the award agreement. The number of market-based rTSRUs awarded represents the target number of shares of common stock that may be earned; however, the actual number of shares that may be earned ranges from 0% to 200% of the target number. The Company used a Monte Carlo simulation model to estimate that the grant-date fair value of the rTSRUs. Assumptions and estimates utilized in the calculation of the fair value of the rTSRUs include the risk-free interest rate, dividend yield, expected volatility based on the historical volatility of publicly traded peer companies and the remaining performance period of the award. The table below sets forth the weighted average grant date fair value assumptions used to value the rTSRUs: Years Ended 2016 2015 Risk-free interest rate 0.94 % 0.61 % Dividend yield 0 % 0 % Expected volatility 65 % 56 % Remaining performance period (years) 1.93 1.86 Estimated fair value per share of rTSRUs granted $ 35.70 $ 26.51 The following table summarizes restricted stock unit activity for the year ended September 30, 2016 based on a target achievement of 100%: Performance Weighted Relative Total Weighted Unvested at September 30, 2015 20,900 $ 35.90 20,900 $ 26.51 Granted 27,625 $ 32.04 27,625 $ 35.70 Vested — $ — — $ — Cancelled — $ — — $ — Unvested at September 30, 2016 48,525 $ 33.70 48,525 $ 31.74 Stock-Based Compensation Expense The Company recorded stock-based compensation expense for the years ended September 30, 2016, 2015, and 2014 in the following expense categories: Years Ended September 30, 2016 2015 2014 Research and development $ 2,882 $ 1,644 $ 762 General and administrative 6,472 4,194 1,920 $ 9,354 $ 5,838 $ 2,682 As of September 30, 2016, the Company had an aggregate of $22,261 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.2 years.

Net Income Per Share

Net Income Per Share	12 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Net Income Per Share	11. Net Income Per Share Basic and diluted net income per common share was calculated as follows for the years ended September 30, 2016, 2015, and 2014: Years Ended September 30, 2016 2015 2014 Basic net income per common share: Numerator: Net income $ 21,666 $ 78,992 $ 34,438 Denominator: Weighted average common shares outstanding—basic 18,929 18,673 18,355 Net income per common share—basic $ 1.14 $ 4.23 $ 1.88 Diluted net income per common share: Numerator: Net income $ 21,666 $ 78,992 $ 34,438 Denominator: Weighted average common shares outstanding—basic 18,929 18,673 18,355 Dilutive effect of common stock equivalents 295 622 830 Weighted average common shares outstanding—diluted 19,224 19,295 19,185 Net income per common share—diluted $ 1.13 $ 4.09 $ 1.80 Antidilutive common stock equivalents excluded from above 1,475 605 330 The impact of certain common stock equivalents were excluded from the computation of diluted net income per common share attributable to common stockholders for the years ended September 30, 2016, 2015, and 2014, because those options had an anti-dilutive impact due to the assumed proceeds per share using the treasury stock method being greater than the average fair value of the Company’s common shares for those periods. For the years ended September 30, 2016, 2015, and 2014, the following weighted average shares were excluded from the calculation of diluted net income per common share as these awards contain performance and market conditions that would not have been achieved at the reporting date had the measurement period ended on that date: Years Ended September 30, 2016 2015 2014 Unvested restricted stock units 84,672 25,424 — Unvested performance-based options 140,210 153,139 157,027

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Sep. 30, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Leases The Company had an office and laboratory lease that was effective from fiscal 2011 to 2018. During the year ended September 30, 2015, the Company amended the lease to expand the rented space and extend the lease term through September 2022. Payment escalations specified in the lease agreement, as amended, are accrued such that rent expense is recognized on a straight-line basis over the term of occupancy. For the years ended September 30, 2016, 2015, and 2014, the Company recognized rent expense of $2,025, $1,326, and $948, respectively, in the consolidated statements of operations. Future minimum lease payments as of September 30, 2016 are as follows: Years Ended September 30, Operating Capital 2017 $ 2,010 $ 73 2018 2,062 79 2019 2,117 86 2020 2,172 93 2021 2,229 101 Thereafter 2,093 99 Total $ 12,683 $ 531 In connection with the current lease, the Company has an outstanding letter of credit in the amount of $608 as of September 30, 2016 and 2015, collateralized by a money market account. As of September 30, 2016 and 2015 the Company classified the money market account as restricted cash. The amended lease also included a $598 tenant improvement allowance from the landlord, which was accounted for as a capital lease obligation. Intellectual Property Licenses The Company has a non-exclusive intellectual property license agreement with a licensor of research technology. Under the 2012 agreement, the Company was required to pay the third party licensor an upfront license fee and additional fees up to the third anniversary of the agreement. In addition, the Company is required to pay (1) annual maintenance fees for each year that the agreement remains in effect, commencing on the first anniversary of the agreement, in order to maintain the right to use the license, and (2) a one-time fee in each circumstance in which the Company provides the licensed intellectual property to one of its collaborators with the prior consent of the licensor. During the years ended September 30, 2016, 2015, and 2014, the Company paid $120, $200 and $250, respectively, under the agreement. During the year ended September 30, 2013, the Company amended an existing license agreement to extend rights to patents previously licensed for one of its programs for use in its HCV research. Under the license, the Company is obligated to pay milestones totaling up to $5,000 plus low single-digit royalties, for the development and regulatory approval of each HCV product outside of the Company’s collaboration with AbbVie and any other collaboration it may enter into in the future with a partner that has already licensed these patents. During the year ended September 30, 2016, the Company paid a $500 milestone payment under this amended agreement as a result of the Company’s filing to commence clinical development of its cyclophilin inhibitor candidate which was recorded as a research and development expense. There were no such payments made during the years ended September 30, 2015 and 2014. Litigation and Contingencies Related to Use of Intellectual Property From time to time, the Company may become subject to legal proceedings, claims and litigation arising in the ordinary course of business. The Company currently is not a party to any threatened or pending litigation. However, third parties might allege that the Company or its collaborators are infringing their patent rights or that the Company is otherwise violating their intellectual property rights. Such third parties may resort to litigation against the Company or its collaborators, which the Company has agreed to indemnify. With respect to some of these patents, the Company expects that it will be required to obtain licenses and could be required to pay license fees or royalties, or both. These licenses may not be available on acceptable terms, or at all. A costly license, or inability to obtain a necessary license, would have a material adverse effect on the Company’s financial condition, results of operations or cash flows. The Company accrues contingent liabilities when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. Indemnification Agreements In the ordinary course of business, the Company may provide indemnifications of varying scope and terms to customers, vendors, lessors, business partners, and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements, from services to be provided by the Company, or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. In addition, the Company maintains officers and directors insurance coverage. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has not accrued any liabilities related to such obligations in its financial statements as of September 30, 2016 and 2015.

Income Taxes

Income Taxes	12 Months Ended
	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Income Taxes	13. Income Taxes Income before income taxes for all periods presented is from domestic operations, which are the Company’s only operations. During the years ended September 30, 2016, 2015, and 2014, the Company recorded an income tax (expense) benefit as follows: Years Ended September 30, 2016 2015 2014 Current income tax (expense) benefit: Federal $ (12,233 ) $ (35,040 ) $ — State (956 ) (2,172 ) (151 ) Deferred income tax (expense) benefit: Federal 2,136 (7,371 ) 13,048 State 159 (1,880 ) 2,273 $ (10,894 ) $ (46,463 ) $ 15,170 For the years ended September 30, 2016 and 2015, the Company recorded an income tax provision of $(10,894) and $(46,463), respectively. During fiscal 2014, the Company reassessed the need for a valuation allowance against its deferred tax assets. The Company concluded from the reassessment that it was more likely than not that the Company would be able to realize its deferred tax assets primarily as a result of recent profitability and forecasted future profits resulting from future expected milestone payments and on-going royalty payments from its AbbVie collaboration. Accordingly, the Company released its valuation allowance of $22,892 in fiscal 2014. The effective tax rate for the year ended September 30, 2016 was 33.5% and differs from the federal statutory rate of 35.0% primarily due to reinstatement of the federal research and development tax credits which are deductible for tax purposes and included in the Company’s annual effective tax rate. The effective tax rate for the year ended September 30, 2015 was 37.0% and differs from the federal statutory rate of 35.0% primarily due to state income taxes, changes in estimates related to research and development tax credits for fiscal years prior to 2015, and certain expenditures which are permanently not deductible for tax purposes. The effective tax rate benefit for the year ended September 30, 2014 was (78.7%) and differs from the federal statutory rate of 35.0% primarily due to a change in estimate as to the Company’s ability to realize the benefits of its net operating loss carry-forwards and research and development tax credits resulting in a reduction of the Company’s valuation allowance against deferred tax assets in the amount of $22,892, as well as changes in estimates related to the fiscal 2014 research and development tax credits, and certain expenditures which are permanently not deductible for tax purposes. A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Years Ended September 30, 2016 2015 2014 Federal statutory income tax rate 35.0 % 35.0 % 35.0 % State taxes, net of federal benefit 2.7 2.6 5.5 Federal research and development tax credit (3.8 ) (0.1 ) (0.5 ) Change in deferred tax asset valuation allowance — — (118.8 ) Change in statutory rate — 0.3 (2.1 ) Other (0.4 ) (0.8 ) 2.2 Effective income tax rate 33.5 % 37.0 % (78.7 %) In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes Net deferred tax assets as of September 30, 2016 and 2015 consisted of the following: September 30, 2016 2015 Deferred tax assets: Capitalized research and development expenses $ 2,052 $ 2,903 Share-based compensation 4,919 2,129 Other temporary differences 1,498 1,243 Total deferred tax assets 8,469 6,275 Valuation allowance — — Net deferred tax assets 8,469 6,275 Deferred tax liabilities: Depreciation (68 ) (168 ) Unrealized gain (loss) (11 ) (20 ) Total deferred tax liabilities (79 ) (188 ) Net deferred income tax assets (liabilities) $ 8,390 $ 6,087 During the year ended September 30, 2016, the Company utilized all remaining federal and state net operating loss carryforwards and all federal and state research and development tax credits generated during fiscal 2015. During the year ended September 30, 2015, the Company utilized $0 and $8,433 of federal and state net operating loss carryforwards, respectively generated during fiscal 2014. During the year ended September 30, 2015, the Company also utilized $4,481 and $2,214 of federal and state research and development tax credit and other credit carryforwards, respectively, generated during 2014. The Company files tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s tax years are still open under statute from 2012 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. Uncertain tax positions represent tax positions for which reserves have been established. The Company’s policy is to record interest and penalties related to uncertain tax positions as part of its income tax provision. A reconciliation of the beginning and ending amount of uncertain tax positions is summarized as follows: September 30, 2016 2015 Beginning Balance $ 448 $ — Additions based on tax positions for the current period 294 324 Additions based on tax positions for prior periods 16 124 Reductions for tax positions of prior periods (13 ) — Ending Balance $ 745 $ 448

401(k) Plan

401(k) Plan	12 Months Ended
	Sep. 30, 2016
Postemployment Benefits [Abstract]
401(k) Plan	14. 401(k) Plan The Company has a 401(k) plan. This plan covers substantially all employees who meet minimum age and service requirements. During the years ended September 30, 2016, 2015, and 2014, the Company recognized $513, $382 and $180, respectively, of expense related to its contributions to this plan.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (unaudited)	12 Months Ended
	Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (unaudited)	15. Selected Quarterly Financial Data (unaudited) Quarterly financial information for fiscal 2016 and 2015 is presented in the following table: 2016 Quarter Ended December 31, 2015 March 31, 2016 June 30, 2016 September 30, 2016 Revenue (1) $ 48,445 $ 13,004 $ 13,978 $ 12,841 Operating expenses 12,851 13,569 15,067 15,940 Other income (expense), net 329 472 447 471 Income tax (expense) benefit (9,734 ) (1,552 ) (434 ) 826 Net income (loss) 26,189 (1,645 ) (1,076 ) (1,802 ) Net income (loss) per common share—basic $ 1.39 $ (0.09 ) $ (0.06 ) $ (0.09 ) Net income (loss) per common share—diluted $ 1.37 $ (0.09 ) $ (0.06 ) $ (0.09 ) 2015 Quarter Ended December 31, 2014 March 31, 2015 June 30, 2015 September 30, 2015 Revenue (2) $ 77,498 $ 57,367 $ 11,599 $ 14,416 Operating expenses 7,288 8,806 9,896 10,742 Other income (expense), net 301 210 287 509 Income tax (expense) benefit (28,502 ) (20,018 ) 428 1,629 Net income 42,009 28,753 2,418 5,812 Net income per common share—basic $ 2.26 $ 1.54 $ 0.13 $ 0.30 Net income per common share—diluted $ 2.18 $ 1.49 $ 0.13 $ 0.29 (1) During the first quarter of 2016, the Company recognized $30.0 million in milestone revenue from AbbVie upon achievement of commercialization regulatory approval of AbbVie’s paritaprevir-containing regimen in Japan. (2) During the first quarter of 2015, the Company recognized $75.0 million in milestone revenue from AbbVie upon achievement of commercialization regulatory approval of AbbVie’s paritaprevir-containing regimen in the U.S. During the second quarter of 2015, the Company recognized $50.0 million in milestone revenue from AbbVie upon achievement of commercialization regulatory approval of AbbVie’s paritaprevir-containing regimen in Europe.

Summary of Significant Accoun24

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements include those of the Company and its subsidiary, Enanta Pharmaceuticals Security Corporation, after elimination of all intercompany accounts and transactions. The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”).
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Significant estimates and assumptions reflected in these consolidated financial statements include, but are not limited to, management’s judgments of separate units of accounting and best estimate of selling price of those units of accounting within its revenue arrangements; valuation of warrants, Series 1 nonconvertible preferred stock and stock-based awards; and the accounting for income taxes, including uncertain tax positions and the valuation of net deferred tax assets. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
Cash Equivalents and Marketable Securities	Cash Equivalents and Marketable Securities The Company considers all short-term, highly liquid investments with original maturities of ninety days or less at acquisition date to be cash equivalents. Marketable securities with original maturities of greater than 90 days and remaining maturities of less than one year from the balance sheet date are classified as short-term marketable securities. Marketable securities with remaining maturities of greater than one year from the balance sheet date are classified as long-term marketable securities. The Company classifies all of its marketable securities as available-for-sale. All marketable securities are held with one investment manager. The Company continually evaluates the credit ratings of its investment portfolio and underlying securities. The Company invests in accordance with its investment policy and invests at the date of purchase in securities with a rating of A3 or higher and A—or higher according to Moody’s and S&P, respectively. The Company reports available-for-sale investments at fair value as of each balance sheet date and records any unrealized gains or losses as a component of stockholders’ equity. The cost of securities sold is determined on a specific identification basis, and realized gains and losses are included in other income (expense), net within the consolidated statements of operations. If any adjustment to fair value reflects a decline in the value of the investment, the Company considers available evidence to evaluate the extent to which the decline is “other than temporary” and reduces the investment to fair value through a charge to the consolidated statements of operations. There were no such adjustments necessary during the years ended September 30, 2016, 2015 and 2014.
Restricted Cash	Restricted Cash As of September 30, 2016 and 2015, the Company had an outstanding letter of credit collateralized by a money market account of $608 to the benefit of the landlord of the Company’s current building lease. This amount was classified as long-term restricted cash as of September 30, 2016 and 2015.
Concentration of Credit Risk and of Significant Customers and Suppliers	Concentration of Credit Risk and of Significant Customers and Suppliers Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities, accounts receivable and unbilled receivables. The Company has all cash and investment balances at one accredited financial institution, in amounts that exceed federally insured limits. The Company does not believe that it is subject to unusual credit risk beyond the normal credit risk associated with commercial banking relationships. The Company has historically generated all of its revenue from its collaborative research and license agreements as well as a U.S. government contract (see Note 7). As of September 30, 2016, accounts receivable consisted of amounts due from the Company’s principal collaborator (see Note 7). The Company is completely dependent on third-party manufacturers for product supply for preclinical and clinical research activities in its non-partnered programs. The Company relies and expects to continue to rely exclusively on several manufacturers to supply it with its requirements for the active pharmaceutical ingredients related to these programs. These research programs would be adversely affected by a significant interruption in the supply of its active pharmaceutical ingredients.
Fair Value Measurements	Fair Value Measurements Certain assets and liabilities are carried at fair value under GAAP. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. A fair value hierarchy is based on three levels of inputs which are used to measure fair value, of which the first two levels are considered observable and the last is considered unobservable: • Level 1—Quoted prices in active markets for identical assets or liabilities. • Level 2—Observable inputs (other than Level 1 quoted prices) such as quoted prices in active markets for similar assets or liabilities, quoted prices in markets that are not active for identical or similar assets or liabilities, or other inputs that are observable or can be corroborated by observable market data. • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to determining the fair value of the assets or liabilities, including pricing models, discounted cash flow methodologies and similar techniques. The Company’s instruments that are carried at fair value are cash equivalents, marketable securities and the warrant and Series 1 nonconvertible preferred stock liabilities. The carrying values of accounts receivable and unbilled receivables, accounts payable and accrued expenses approximate their fair value due to the short-term nature of these assets and liabilities.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation or amortization. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Laboratory and office equipment 5 years Leasehold improvements Shorter of life of lease or estimated useful life Purchased software 3 years Computer equipment 3 years Furniture 7 years Expenditures for repairs and maintenance of assets are charged to expense as incurred. Costs of major additions and betterments are capitalized and depreciated on a straight-line basis over their useful lives. Upon retirement or sale, the cost and related accumulated depreciation or amortization of assets disposed are removed from the accounts and any resulting gain or loss is included in income from operations in the consolidated statements of operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company reviews 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. To date, the Company has not recorded any impairment losses on long-lived assets.
Revenue Recognition	Revenue Recognition The Company’s revenue has been generated primarily through collaborative research and license agreements. The terms of these agreements contain multiple deliverables, which may include (i) licenses, (ii) research and development activities, and (iii) participation in joint research and development steering committees. The terms of these agreements may include nonrefundable upfront license fees, payments for research and development activities, payments based upon the achievement of certain milestones, and royalty payments based on product sales derived from the collaboration. In all instances, revenue is recognized only when the price is fixed or determinable, persuasive evidence of an arrangement exists, delivery has occurred or the services have been rendered, collectibility of the resulting receivable is reasonably assured, and the Company has fulfilled its performance obligations under the contract. On October 1, 2011, the Company adopted Accounting Standards Update No. 2009-13, Multiple-Deliverable Revenue Arrangements In determining the separate units of accounting, the Company evaluates whether the license has standalone value to the collaborator based on consideration of the relevant facts and circumstances for each arrangement. Factors considered in this determination include the research and development capabilities of the collaborator and the availability of relevant research expertise in the marketplace. In addition, the Company considers whether or not (i) the collaborator can use the license for its intended purpose without the receipt of the remaining deliverables, (ii) the value of the license is dependent on the undelivered items, and (iii) the collaborator or other vendors can provide the undelivered items. For all periods presented, whenever the Company determines that an element is delivered over a period of time, revenue is recognized using either a proportional performance model or a straight-line model over the period of performance, which is typically the research and development term. Full-time equivalents (“FTEs”) are typically used as the measure of performance. At each reporting period, the Company reassesses its cumulative measure of performance and makes appropriate adjustments, if necessary. The Company recognizes revenue using the proportional performance model whenever the Company can make reasonably reliable estimates of the level of effort required to complete its performance obligations under an arrangement. Revenue recognized under the proportional performance model at each reporting period is determined by multiplying the total expected payments under the contract (excluding royalties and payments contingent upon achievement of milestones) by the ratio of the level of effort incurred to date to the estimated total level of effort required to complete the performance obligations under the arrangement. Revenue is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined using the proportional performance model as of each reporting period. Alternatively, if the Company cannot make reasonably reliable estimates of the level of effort required to complete its performance obligations under an arrangement, then revenue under the arrangement is recognized on a straight-line basis over the period expected to complete the Company’s performance obligations. If and when a contingent milestone payment is earned, the additional consideration to be received is allocated to the separate units of accounting in the arrangement based on their relative selling prices at the inception of the arrangement. Revenue is limited to the lesser of the cumulative amount of payments received or the cumulative amount of revenue earned, as determined on a straight-line basis as of the period end date. If the Company cannot reasonably estimate when its performance obligation period ends, then revenue is deferred until the Company can reasonably estimate when the performance obligation period ends. Royalty revenue is recognized based on contractual terms when reported sales are reliably measurable and collectibility is reasonably assured, provided that there are no performance obligations remaining. During the years ended September 30, 2016, 2015 and 2014, the Company also generated revenue from a government contract, under which the Company was reimbursed for certain allowable costs for the funded project. Revenue from the government contract was recognized when the related service was performed. The related costs incurred by the Company under the government contract were included in research and development expenses in the consolidated statements of operations. Amounts received prior to satisfying all revenue recognition criteria are recorded as deferred revenue in the accompanying consolidated balance sheets. Amounts not expected to be recognized as revenue within the next twelve months of the consolidated balance sheet date are classified as long-term deferred revenue. In the event that a collaborative research and license agreement is terminated and the Company then has no further performance obligations, the Company recognizes as revenue any amounts that had not previously been recorded as revenue but were classified as deferred revenue at the date of such termination.
Research and Development Costs	Research and Development Costs Research and development costs are expensed as incurred. Included in research and development costs are wages, stock-based compensation and benefits of employees, third-party license fees and other operational costs related to the Company’s research and development activities, including facility-related expenses and external costs of outside contractors engaged to conduct both preclinical and clinical studies and manufacture quantities of product for preclinical and clinical studies. The Company also includes in research and development expense the costs to complete the Company’s obligations under research collaborations.
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.
Stock-Based Compensation	Stock-Based Compensation The Company measures all stock options and other stock-based awards granted to employees at fair value on the date of grant. The Company uses the Black-Scholes option-pricing model in the valuation of its stock options. The fair value of performance-based awards is based on intrinsic value of the stock on the date of grant. The Company uses the Monte-Carlo simulation in order to calculate the fair value of the market-based awards. The fair value of options is recognized as stock-based compensation 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-based and market-based conditions. The Company records stock-based compensation expense related to performance-based awards when the performance-based targets are probable of being achieved. 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.
Income Taxes	Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more-likely-than-not to be sustained, the tax position is then assessed to determine the amount of benefit to be recognized in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. As discussed in Note 13, the Company adopted ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes
Net Income per Share	Net Income per Share Basic net income per common share is computed by dividing the net income by the weighted average number of shares of common stock outstanding for the period. Diluted net income per common share is computed by dividing net income by the weighted average number of common shares outstanding for the period, including potential dilutive common shares assuming the dilutive effect of outstanding stock options, unvested restricted stock units and outstanding warrants. Market-based awards are included in diluted earnings per common share to the extent they would have vested if the period end date was the market criteria measurement date.
Segment Data	Segment Data The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company is a biotechnology company focused on discovering and developing small molecule drugs for the treatment of viral infections and liver diseases. Revenue is generated exclusively from transactions occurring with partners located in the United States and all assets are held in the United States.
Comprehensive Income	Comprehensive Income Comprehensive income includes net income as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. The Company’s only element of other comprehensive income is unrealized gains and losses on available-for-sale marketable securities.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (the “FASB”) issued ASU No. 2014-09, Revenue from Contracts with Customers , Revenue from Contract with Customers: Principal versus Agent Considerations Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients. In February 2016, the FASB issued ASU No. 2016-02, Leases In March 2016, the FASB issued ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash Other accounting standards that have been issued or proposed by the FASB or other standards-setting bodies that do not require adoption until a future date are not expected to have a material impact on the Company’s consolidated financial statements upon adoption.

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Sep. 30, 2016
Accounting Policies [Abstract]
Property and Equipment Estimated Useful Lives	Property and equipment are stated at cost less accumulated depreciation or amortization. Depreciation and amortization expense is recognized using the straight-line method over the following estimated useful lives: Laboratory and office equipment 5 years Leasehold improvements Shorter of life of lease or estimated useful life Purchased software 3 years Computer equipment 3 years Furniture 7 years

Fair Value of Financial Asset26

Fair Value of Financial Assets and Liabilities (Tables)	12 Months Ended
	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis	The following tables present information about the Company’s financial assets and liabilities that were subject to fair value measurement on a recurring basis as of September 30, 2016 and 2015 and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value: Fair Value Measurements at September 30, 2016 Using: Level 1 Level 2 Level 3 Total Assets: U.S. Treasury notes $ 69,608 $ — $ — $ 69,608 Cash equivalents 15,295 — — 15,295 Corporate bonds — 76,073 — 76,073 Commercial paper — 49,900 — 49,900 U.S. Agency bonds — 30,045 — 30,045 $ 84,903 $ 156,018 $ — $ 240,921 Liabilities: Warrant liability $ — $ — $ 1,251 $ 1,251 Series 1 nonconvertible preferred stock — — 159 159 $ — $ — $ 1,410 $ 1,410 Fair Value Measurements at September 30, 2015 Using: Level 1 Level 2 Level 3 Total Assets: Cash equivalents $ 21,327 $ — $ — $ 21,327 Corporate bonds — 151,020 — 151,020 U.S. Agency bonds — 36,697 — 36,697 $ 21,327 $ 187,717 $ — $ 209,044 Liabilities: Warrant liability $ — $ — $ 1,276 $ 1,276 Series 1 nonconvertible preferred stock — — 163 163 $ — $ — $ 1,439 $ 1,439
Fair Value Measurements of the Company's Warrant Liability and Series 1 Nonconvertible Preferred Stock	The recurring Level 3 fair value measurements of the Company’s warrant liability and Series 1 nonconvertible preferred stock using probability-weighted discounted cash flow include the following significant unobservable inputs: Range (Weighted Average) Unobservable Input 2016 2015 Warrant liability and Series 1 nonconvertible preferred stock Probabilities of payout Periods in which payout is expected to occur Discount rate 0% - 60% 2017 - 2018 4.50% 5% - 60% 2016 - 2017 4.25%
Rollforward of Aggregate Fair Values of Warrants and Outstanding Series 1 Nonconvertible Preferred Stock	The following table provides a rollforward of the aggregate fair values of the Company’s warrants for the purchase of Series 1 nonconvertible preferred stock and the outstanding Series 1 nonconvertible preferred stock for which fair value is determined by Level 3 inputs: Warrant Series 1 Balance, September 30, 2013 $ 1,620 $ — Warrants exercised (206 ) — Series 1 nonconvertible preferred stock issued — 206 Increase (decrease) in fair value 170 (4 ) Balance, September 30, 2014 $ 1,584 $ 202 Decrease in fair value (308 ) (39 ) Balance, September 30, 2015 $ 1,276 $ 163 Decrease in fair value (25 ) (4 ) Balance, September 30, 2016 $ 1,251 $ 159

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of September 30, 2016 and 2015, the fair value of available-for-sale marketable securities, by type of security, was as follows: September 30, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate bonds $ 76,077 $ 27 $ (31 ) $ 76,073 U.S. Treasury notes 69,579 38 (9 ) 69,608 Commercial paper 49,900 — — 49,900 U.S. Agency bonds 30,040 15 (10 ) 30,045 $ 225,596 $ 80 $ (50 ) $ 225,626 September 30, 2015 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Corporate bonds $ 151,012 $ 77 $ (69 ) $ 151,020 U.S. Agency bonds 36,652 45 — 36,697 $ 187,664 $ 122 $ (69 ) $ 187,717

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Sep. 30, 2016
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment	Property and equipment consisted of the following as of September 30, 2016 and 2015: September 30, 2016 2015 Laboratory and office equipment $ 7,440 $ 5,907 Leasehold improvements 3,715 1,290 Purchased software 705 443 Furniture 551 496 Construction in progress 504 1,889 Computer equipment 187 137 13,102 10,162 Less: Accumulated depreciation and amortization (5,098 ) (4,276 ) $ 8,004 $ 5,886

Accrued Expenses, Other Curre29

Accrued Expenses, Other Current Liabilities and Other Long-Term Liabilities (Tables)	12 Months Ended
	Sep. 30, 2016
Payables and Accruals [Abstract]
Accrued Expenses, Other Current Liabilities and Other Long-term Liabilities	Accrued expenses, other current liabilities and other long-term liabilities consisted of the following as of September 30, 2016 and 2015: September 30, 2016 2015 Accrued expenses and other current liabilities: Accrued payroll and related expenses $ 2,384 $ 1,622 Accrued preclinical and clinical expenses 899 237 Accrued vendor manufacturing 441 18 Accrued professional fees 393 338 Capital lease obligation 73 69 Accrued fixed assets 2 1,307 Accrued other 320 371 $ 4,512 $ 3,962 Other long-term liabilities: Uncertain tax positions $ 745 $ 448 Accrued rent expense 696 628 Capital lease obligation 458 529 Asset retirement obligation 143 108 $ 2,042 $ 1,713

Series 1 Nonconvertible Prefe30

Series 1 Nonconvertible Preferred Stock and Warrants (Tables)	12 Months Ended
	Sep. 30, 2016
Debt Disclosure [Abstract]
Summary of Outstanding Warrants	The following table summarizes the Company’s outstanding warrants: Series 1 Nonconvertible Preferred Stock Warrants Exercisable Weighted Average Exercise Price Outstanding, as of September 30, 2013 1,999,989 $ 0.01 Granted — — Expired — — Exercised (225,408 ) $ 0.01 Outstanding, as of September 30, 2014 1,774,581 $ 0.01 Granted — — Expired — — Exercised — — Outstanding, as of September 30, 2015 1,774,581 $ 0.01 Granted — — Expired — — Exercised — — Outstanding, as of September 30, 2016 1,774,581 $ 0.01

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	12 Months Ended
	Sep. 30, 2016
Data Used to Determine Value of Stock Option Grants, Presented on Weighted Average Basis	The relevant data used to determine the value of the stock option awards are as follows, presented on a weighted average basis: Years Ended September 30, 2016 2015 2014 Risk-free interest rate 1.77 % 1.85 % 1.89 % Expected term (in years) 6.10 6.03 6.06 Expected volatility 70 % 74 % 75 % Expected dividends 0 % 0 % 0 %
Stock Option Activity	The following table summarizes stock option activity, including aggregate intrinsic value for the year ended September 30, 2016: Shares Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term in years Aggregate Intrinsic Value Outstanding as of September 30, 2015 1,752,010 $ 23.76 7.2 $ 25,093 Granted 509,680 30.31 Exercised (319,076 ) 3.22 Forfeited (47,158 ) 33.04 Outstanding, September 30, 2016 1,895,456 $ 28.75 7.6 $ 7,369 Options vested and expected to vest as of September 30, 2016 1,772,221 $ 29.65 7.6 $ 5,958 Options exercisable as of September 30, 2016 957,141 $ 25.52 6.9 $ 5,727
Summary of Additional Exercise and Grant Date Information	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. The following table summarizes additional exercise and grant date information: Years Ended September 30, 2016 2015 2014 Aggregate intrinsic value of stock options exercised $ 7,705 $ 4,827 $ 3,219 Proceeds from stock options exercised $ 1,026 $ 726 $ 1,054 Weighted average grant date fair value of options granted (per share) $ 19.12 $ 27.77 $ 19.98
Summary of Restricted Stock Units Activity	The following table summarizes restricted stock unit activity for the year ended September 30, 2016 based on a target achievement of 100%: Performance Weighted Relative Total Weighted Unvested at September 30, 2015 20,900 $ 35.90 20,900 $ 26.51 Granted 27,625 $ 32.04 27,625 $ 35.70 Vested — $ — — $ — Cancelled — $ — — $ — Unvested at September 30, 2016 48,525 $ 33.70 48,525 $ 31.74
Stock-Based Compensation Expense	The Company recorded stock-based compensation expense for the years ended September 30, 2016, 2015, and 2014 in the following expense categories: Years Ended September 30, 2016 2015 2014 Research and development $ 2,882 $ 1,644 $ 762 General and administrative 6,472 4,194 1,920 $ 9,354 $ 5,838 $ 2,682
Relative Total Shareholder Return Units [Member]
Data Used to Determine Value of Stock Option Grants, Presented on Weighted Average Basis	The table below sets forth the weighted average grant date fair value assumptions used to value the rTSRUs: Years Ended 2016 2015 Risk-free interest rate 0.94 % 0.61 % Dividend yield 0 % 0 % Expected volatility 65 % 56 % Remaining performance period (years) 1.93 1.86 Estimated fair value per share of rTSRUs granted $ 35.70 $ 26.51

Net Income Per Share (Tables)

Net Income Per Share (Tables)	12 Months Ended
	Sep. 30, 2016
Earnings Per Share [Abstract]
Basic and Diluted Net Income Per Common Share	Basic and diluted net income per common share was calculated as follows for the years ended September 30, 2016, 2015, and 2014: Years Ended September 30, 2016 2015 2014 Basic net income per common share: Numerator: Net income $ 21,666 $ 78,992 $ 34,438 Denominator: Weighted average common shares outstanding—basic 18,929 18,673 18,355 Net income per common share—basic $ 1.14 $ 4.23 $ 1.88 Diluted net income per common share: Numerator: Net income $ 21,666 $ 78,992 $ 34,438 Denominator: Weighted average common shares outstanding—basic 18,929 18,673 18,355 Dilutive effect of common stock equivalents 295 622 830 Weighted average common shares outstanding—diluted 19,224 19,295 19,185 Net income per common share—diluted $ 1.13 $ 4.09 $ 1.80 Antidilutive common stock equivalents excluded from above 1,475 605 330
Weighted Average Shares Excluded From Calculation of Diluted Net Income Per Share Attributable to Common Stockholders	For the years ended September 30, 2016, 2015, and 2014, the following weighted average shares were excluded from the calculation of diluted net income per common share as these awards contain performance and market conditions that would not have been achieved at the reporting date had the measurement period ended on that date: Years Ended September 30, 2016 2015 2014 Unvested restricted stock units 84,672 25,424 — Unvested performance-based options 140,210 153,139 157,027

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Sep. 30, 2016
Commitments and Contingencies Disclosure [Abstract]
Future Minimum Lease Payments	Future minimum lease payments as of September 30, 2016 are as follows: Years Ended September 30, Operating Capital 2017 $ 2,010 $ 73 2018 2,062 79 2019 2,117 86 2020 2,172 93 2021 2,229 101 Thereafter 2,093 99 Total $ 12,683 $ 531

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Summary of Income Tax (Expense) Benefit	During the years ended September 30, 2016, 2015, and 2014, the Company recorded an income tax (expense) benefit as follows: Years Ended September 30, 2016 2015 2014 Current income tax (expense) benefit: Federal $ (12,233 ) $ (35,040 ) $ — State (956 ) (2,172 ) (151 ) Deferred income tax (expense) benefit: Federal 2,136 (7,371 ) 13,048 State 159 (1,880 ) 2,273 $ (10,894 ) $ (46,463 ) $ 15,170
Reconciliation of Income Tax Provision at Federal Statutory Income Tax Rate and Effective Tax Rate	A reconciliation of the U.S. federal statutory income tax rate to the Company’s effective income tax rate is as follows: Years Ended September 30, 2016 2015 2014 Federal statutory income tax rate 35.0 % 35.0 % 35.0 % State taxes, net of federal benefit 2.7 2.6 5.5 Federal research and development tax credit (3.8 ) (0.1 ) (0.5 ) Change in deferred tax asset valuation allowance — — (118.8 ) Change in statutory rate — 0.3 (2.1 ) Other (0.4 ) (0.8 ) 2.2 Effective income tax rate 33.5 % 37.0 % (78.7 %)
Net Deferred Tax Assets and Liabilities	Net deferred tax assets as of September 30, 2016 and 2015 consisted of the following: September 30, 2016 2015 Deferred tax assets: Capitalized research and development expenses $ 2,052 $ 2,903 Share-based compensation 4,919 2,129 Other temporary differences 1,498 1,243 Total deferred tax assets 8,469 6,275 Valuation allowance — — Net deferred tax assets 8,469 6,275 Deferred tax liabilities: Depreciation (68 ) (168 ) Unrealized gain (loss) (11 ) (20 ) Total deferred tax liabilities (79 ) (188 ) Net deferred income tax assets (liabilities) $ 8,390 $ 6,087
Schedule of Reconciliation of Uncertain Tax Positions	A reconciliation of the beginning and ending amount of uncertain tax positions is summarized as follows: September 30, 2016 2015 Beginning Balance $ 448 $ — Additions based on tax positions for the current period 294 324 Additions based on tax positions for prior periods 16 124 Reductions for tax positions of prior periods (13 ) — Ending Balance $ 745 $ 448

Selected Quarterly Financial 35

Selected Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	Quarterly financial information for fiscal 2016 and 2015 is presented in the following table: 2016 Quarter Ended December 31, 2015 March 31, 2016 June 30, 2016 September 30, 2016 Revenue (1) $ 48,445 $ 13,004 $ 13,978 $ 12,841 Operating expenses 12,851 13,569 15,067 15,940 Other income (expense), net 329 472 447 471 Income tax (expense) benefit (9,734 ) (1,552 ) (434 ) 826 Net income (loss) 26,189 (1,645 ) (1,076 ) (1,802 ) Net income (loss) per common share—basic $ 1.39 $ (0.09 ) $ (0.06 ) $ (0.09 ) Net income (loss) per common share—diluted $ 1.37 $ (0.09 ) $ (0.06 ) $ (0.09 ) 2015 Quarter Ended December 31, 2014 March 31, 2015 June 30, 2015 September 30, 2015 Revenue (2) $ 77,498 $ 57,367 $ 11,599 $ 14,416 Operating expenses 7,288 8,806 9,896 10,742 Other income (expense), net 301 210 287 509 Income tax (expense) benefit (28,502 ) (20,018 ) 428 1,629 Net income 42,009 28,753 2,418 5,812 Net income per common share—basic $ 2.26 $ 1.54 $ 0.13 $ 0.30 Net income per common share—diluted $ 2.18 $ 1.49 $ 0.13 $ 0.29 (1) During the first quarter of 2016, the Company recognized $30.0 million in milestone revenue from AbbVie upon achievement of commercialization regulatory approval of AbbVie’s paritaprevir-containing regimen in Japan. (2) During the first quarter of 2015, the Company recognized $75.0 million in milestone revenue from AbbVie upon achievement of commercialization regulatory approval of AbbVie’s paritaprevir-containing regimen in the U.S. During the second quarter of 2015, the Company recognized $50.0 million in milestone revenue from AbbVie upon achievement of commercialization regulatory approval of AbbVie’s paritaprevir-containing regimen in Europe.

Summary of Significant Accoun36

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Summary of Accounting and Financial Policies [Line Items]
Adjustment to fair value	$ 0	$ 0	$ 0
Cash and investment restricted as a collateral for outstanding letter of credit	608,000	608,000
Money Market Funds [Member]
Summary of Accounting and Financial Policies [Line Items]
Cash and investment restricted as a collateral for outstanding letter of credit	$ 608,000	$ 608,000
Maximum [Member]
Summary of Accounting and Financial Policies [Line Items]
Short-term investments maturity period	P90D
Short-term marketable securities maturity period	1 year
Minimum [Member]
Summary of Accounting and Financial Policies [Line Items]
Short-term marketable securities maturity period	90 days
Long-term marketable securities maturity period	1 year
Income tax benefit recognition, percentage of likelihood of being realized upon settlement	50.00%

Summary of Significant Accoun37

Summary of Significant Accounting Policies - Property and Equipment Estimated Useful Lives (Detail)	12 Months Ended
	Sep. 30, 2016
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	5 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Leasehold improvements useful life	Shorter of life of lease or estimated useful life
Purchased Software [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	3 years
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	3 years
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property plant and equipment useful life	7 years

Fair Value of Financial Asset38

Fair Value of Financial Assets and Liabilities - Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Liabilities:
Liabilities, Fair Value Disclosure, Total	$ 1,410	$ 1,439
Assets:
Cash equivalents	15,295	21,327
Available for sale securities, fair value	225,626	187,717
Assets, Fair Value Disclosure, Total	240,921	209,044
Level 1 [Member]
Assets:
Cash equivalents	15,295	21,327
Assets, Fair Value Disclosure, Total	84,903	21,327
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	156,018	187,717
Level 3 [Member]
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,410	1,439
Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	159	163	$ 202
Series 1 Nonconvertible Preferred Stock [Member] | Level 3 [Member]
Liabilities:
Liabilities	159	163
Warrant Liability [Member]
Liabilities:
Liabilities	1,251	1,276	$ 1,584	$ 1,620
Warrant Liability [Member] | Level 3 [Member]
Liabilities:
Liabilities	1,251	1,276
Warrant Liability [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,251	1,276
U.S. Treasury Notes [Member]
Assets:
Available for sale securities, fair value	69,608
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Available for sale securities, fair value	69,608
Corporate Bonds [Member]
Assets:
Available for sale securities, fair value	76,073	151,020
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Available for sale securities, fair value	76,073	151,020
Commercial Paper [Member]
Assets:
Available for sale securities, fair value	49,900
Commercial Paper [Member] | Level 2 [Member]
Assets:
Available for sale securities, fair value	49,900
U.S. Agency Bonds [Member]
Assets:
Available for sale securities, fair value	30,045	36,697
U.S. Agency Bonds [Member] | Level 2 [Member]
Assets:
Available for sale securities, fair value	$ 30,045	$ 36,697

Fair Value of Financial Asset39

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0	$ 0

Fair Value of Financial Asset40

Fair Value of Financial Assets and Liabilities - Fair Value Measurements of the Company's Warrant Liability and Series 1 Nonconvertible Preferred Stock (Detail) - Warrant Liability [Member] - Series 1 Nonconvertible Preferred Stock [Member]	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Periods in which payout is expected to occur, beginning	2,017	2,016
Periods in which payout is expected to occur, ending	2,018	2,017
Discount rate	4.50%	4.25%
Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Probabilities of payout	0.00%	5.00%
Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Probabilities of payout	60.00%	60.00%

Fair Value of Financial Asset41

Fair Value of Financial Assets and Liabilities - Rollforward of Aggregate Fair Values of Warrants (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Series 1 Nonconvertible Preferred Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 163	$ 202
Series 1 nonconvertible preferred stock issued			$ 206
Increase (decrease) in fair value	(4)	(39)	(4)
Ending Balance	159	163	202
Warrant Liability [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,276	1,584	1,620
Warrants exercised			(206)
Increase (decrease) in fair value	(25)	(308)	170
Ending Balance	1,251	1,276	$ 1,584
Warrant Liability [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,276
Ending Balance	$ 1,251	$ 1,276

Marketable Securities - Fair Va

Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 225,596	$ 187,664
Gross Unrealized Gains	80	122
Gross Unrealized Losses	(50)	(69)
Fair Value	225,626	187,717
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	76,077	151,012
Gross Unrealized Gains	27	77
Gross Unrealized Losses	(31)	(69)
Fair Value	76,073	151,020
U.S. Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,040	36,652
Gross Unrealized Gains	15	45
Gross Unrealized Losses	(10)
Fair Value	30,045	$ 36,697
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,900
Fair Value	49,900
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,579
Gross Unrealized Gains	38
Gross Unrealized Losses	(9)
Fair Value	$ 69,608

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Marketable securities maturing in greater than one year, aggregate fair value	$ 32,119	$ 64,238
Short Term Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	Within one year
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	Between one and three years
U.S. Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	Between one and three years
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	Between one and three years

Property and Equipment - Schedu

Property and Equipment - Schedule of Property and Equipment (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Property, Plant and Equipment [Line Items]
Property and equipment	$ 13,102	$ 10,162
Less: Accumulated depreciation and amortization	(5,098)	(4,276)
Property and equipment, net	8,004	5,886
Laboratory and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	7,440	5,907
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	3,715	1,290
Purchased Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	705	443
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	187	137
Furniture [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	551	496
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 504	$ 1,889

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Property, Plant and Equipment [Abstract]
Depreciation and amortization expense	$ 1,661	$ 639	$ 352

Accrued Expenses, Other Curre46

Accrued Expenses, Other Current Liabilities and Other Long-Term Liabilities - Accrued Expenses, Other Current Liabilities and Other Long-term Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Accrued expenses and other current liabilities:
Accrued payroll and related expenses	$ 2,384	$ 1,622
Accrued preclinical and clinical expenses	899	237
Accrued vendor manufacturing	441	18
Accrued professional fees	393	338
Capital lease obligation	73	69
Accrued fixed assets	2	1,307
Accrued other	320	371
Accrued expenses	4,512	3,962
Other long-term liabilities:
Uncertain tax positions	745	448
Accrued rent expense	696	628
Capital lease obligation	458	529
Asset retirement obligation	143	108
Other long-term liabilities	$ 2,042	$ 1,713

Collaboration Agreements - Addi

Collaboration Agreements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration agreement date	Nov. 27, 2006
Milestone revenue payments received	$ 30,000	$ 125,000	$ 40,000
Collaboration agreement date	Sep. 30, 2011
AbbVie [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Agreement continuation period	10 years
AbbVie [Member] | Paritaprevir 3-DAA Regimen [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of net sales to be allocated	30.00%
AbbVie [Member] | Paritaprevir 2-DAA Regimen [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of net sales to be allocated	45.00%
AbbVie [Member] | Glecaprevir 2-DAA Regimen [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of net sales to be allocated	50.00%
AbbVie [Member] | Additional Funding Agreement Terms [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Potential future milestone	$ 80,000
AbbVie [Member] | Milestone Payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone revenue payments received	30,000	125,000	40,000
AbbVie [Member] | Milestone Payments and Royalties [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Proceed received to fund research activities and preferred stock financing	394,000
National Institutes of Health, National Institute of Allergy and Infectious Diseases Contract [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from grants	576	$ 1,803	$ 7,741
Funding received for research and development	$ 20,637

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Mar. 26, 2013	Sep. 30, 2015
Class of Stock [Line Items]
Shares issued in initial public offering	4,600,000
Proceeds from initial public offering, net of underwriting discounts and commissions	$ 59,892
Shares issued, price per share	$ 14
Treasury stock, shares retired but unissued		208,816,000
Conversion of Redeemable Convertible Preferred Stock and Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Shares of common stock issued upon conversion of preferred stock	11,656,875

Series 1 Nonconvertible Prefe49

Series 1 Nonconvertible Preferred Stock and Warrants - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013	Nov. 30, 2010
Class of Stock [Line Items]
Fair value of the Series 1 nonconvertible preferred stock	$ 159,000	$ 163,000
Warrant Liability [Member]
Class of Stock [Line Items]
Fair value of series 1 nonconvertible preferred stock warrants	$ 1,251,000	1,276,000	$ 1,584,000	$ 1,620,000
Series 1 Nonconvertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Shares authorized	1,999,989
Par value of outstanding convertible preferred stock	$ 0.01
Per share value adjusted for stock dividend	1
Preferred stock redemption price per share	$ 1
Redemption period	60 days
Number of shares issuable upon exercise of the warrants					1,999,989
Warrant expiration date	Oct. 4, 2017
Transfer of assets upon exercise of warrants maximum amount					$ 2,000,000
Change in fair value of warrants	$ 29,000	347,000	(166,000)
Fair value of the Series 1 nonconvertible preferred stock	159,000	163,000
Fair value of series 1 nonconvertible preferred stock warrants	159,000	163,000	$ 202,000
Series 1 Nonconvertible Preferred Stock [Member] | Warrant Liability [Member]
Class of Stock [Line Items]
Fair value of series 1 nonconvertible preferred stock warrants	$ 1,251,000	$ 1,276,000

Series 1 Nonconvertible Prefe50

Series 1 Nonconvertible Preferred Stock and Warrants - Summary of Outstanding Warrants (Detail) - Series 1 Nonconvertible Preferred Stock [Member] - $ / shares	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Preferred Stock And Warrant [Line Items]
Beginning Balance	1,774,581	1,774,581	1,999,989
Granted	0	0	0
Expired	0	0	0
Exercised			(225,408)
Ending Balance	1,774,581	1,774,581	1,774,581
Weighted Average Exercise Price Outstanding, Beginning Balance	$ 0.01	$ 0.01	$ 0.01
Weighted Average Exercise Price Outstanding, Exercised			0.01
Weighted Average Exercise Price Outstanding, Ending Balance	$ 0.01	$ 0.01	$ 0.01

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($)	Oct. 01, 2015	Mar. 31, 2013	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield			0.00%	0.00%	0.00%
Shares awarded					0
Compensation expenses			$ 9,354,000	$ 5,838,000	$ 2,682,000
Aggregate of unrecognized stock-based compensation cost			$ 22,261,000
Weighted average recognition period			2 years 2 months 12 days
Performance-Based Options [Member] | Certain Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded		167,052
Aggregate grant date fair value of award		$ 2,479,000
Compensation expenses			$ 620,000	0	206,000
Relative Total Shareholder Return Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded			27,625
Relative Total Shareholder Return Units [Member] | Certain Executives [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares range percentage			200.00%
Relative Total Shareholder Return Units [Member] | Certain Executives [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares range percentage			0.00%
PSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares awarded			27,625
PSU [Member] | Certain Executives [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, requisite service period			2 years
Compensation expenses			$ 0	$ 0	$ 0
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Target to achieve, percentage			100.00%
2012 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of potential common stock shares to be issued			3.00%
Common stock shares authorized	561,505
Shares available for future grant			487,906
2012 Equity Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares authorized			2,088,167
2012 Equity Incentive Plan [Member] | Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, requisite service period			4 years
Options granted, expiration period			10 years
2012 Equity Incentive Plan [Member] | Employees [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, vesting period			4 years
2012 Equity Incentive Plan [Member] | Employees [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, vesting period			1 year
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant			185,614
Shares issued			0

Stock-Based Awards - Data Used

Stock-Based Awards - Data Used to Determine Value of Stock Option Grants, Presented on Weighted Average Basis (Detail) - $ / shares	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Share Based Payment Award Stock Options Valuation Assumptions [Line Items]
Risk-free interest rate	1.77%	1.85%	1.89%
Expected volatility	70.00%	74.00%	75.00%
Expected term (in years)	6 years 1 month 6 days	6 years 11 days	6 years 22 days
Expected dividends	0.00%	0.00%	0.00%
Relative Total Shareholder Return Units [Member]
Share Based Payment Award Stock Options Valuation Assumptions [Line Items]
Estimated fair value per share of rTSRUs granted	$ 35.70
Executive Officer [Member] | Relative Total Shareholder Return Units [Member]
Share Based Payment Award Stock Options Valuation Assumptions [Line Items]
Risk-free interest rate	0.94%	0.61%
Dividend yield	0.00%	0.00%
Expected volatility	65.00%	56.00%
Expected term (in years)	1 year 11 months 5 days	1 year 10 months 10 days
Estimated fair value per share of rTSRUs granted	$ 35.70	$ 26.51

Stock-Based Awards - Stock Opti

Stock-Based Awards - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Options
Outstanding as of beginning of period	1,752,010
Granted	509,680
Exercised	(319,076)
Forfeited	(47,158)
Outstanding as of end of period	1,895,456	1,752,010
Options vested and expected to vest as of end of period	1,772,221
Options exercisable as of end of period	957,141
Weighted Average Exercise Price
Granted	$ 30.31
Exercised	3.22
Forfeited	33.04
Outstanding	28.75	$ 23.76
Options vested and expected to vest as of end of period	29.65
Options exercisable as of end of period	$ 25.52
Weighted Average Remaining Contractual Term
Outstanding as of end of period	7 years 7 months 6 days	7 years 2 months 12 days
Options vested and expected to vest as of end period	7 years 7 months 6 days
Options exercisable as of end of period	6 years 10 months 24 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value	$ 7,369	$ 25,093
Options vested and expected to vest as of end of period	5,958
Options exercisable as of end of period	5,727
Aggregate intrinsic value of stock options exercised	7,705	4,827	$ 3,219
Proceeds from stock options exercised	$ 1,026	$ 726	$ 1,054
Weighted average grant date fair value of options granted (per share)	$ 19.12	$ 27.77	$ 19.98

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Restricted Stock Units Activity (Detail) - $ / shares	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted, shares		0
PSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance, shares	20,900
Restricted stock units granted, shares	27,625
Restricted stock units vested , shares	0
Restricted stock units cancelled,value	0
Unvested ending balance,value	48,525
Unvested beginning balance,value	$ 35.90
Restricted stock units granted,value	32.04
Restricted stock units vested,value	0
Restricted stock units cancelled,value	0
Unvested ending balance,value	$ 33.70
Relative Total Shareholder Return Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance, shares	20,900
Restricted stock units granted, shares	27,625
Restricted stock units vested , shares	0
Restricted stock units cancelled,value	0
Unvested ending balance,value	48,525
Unvested beginning balance,value	$ 26.51
Restricted stock units granted,value	35.70
Restricted stock units vested,value	0
Restricted stock units cancelled,value	0
Unvested ending balance,value	$ 31.74

Stock-Based Awards - Stock-Base

Stock-Based Awards - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 9,354	$ 5,838	$ 2,682
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	2,882	1,644	762
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 6,472	$ 4,194	$ 1,920

Net Income Per Share - Basic an

Net Income Per Share - Basic and Diluted Net Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Basic net income per common share:
Net income	$ (1,802)	$ (1,076)	$ (1,645)	$ 26,189	$ 5,812	$ 2,418	$ 28,753	$ 42,009	$ 21,666	$ 78,992	$ 34,438
Weighted average common shares outstanding-basic									18,929	18,673	18,355
Net income per common share-basic	$ (0.09)	$ (0.06)	$ (0.09)	$ 1.39	$ 0.30	$ 0.13	$ 1.54	$ 2.26	$ 1.14	$ 4.23	$ 1.88
Diluted net income per common share:
Net income	$ (1,802)	$ (1,076)	$ (1,645)	$ 26,189	$ 5,812	$ 2,418	$ 28,753	$ 42,009	$ 21,666	$ 78,992	$ 34,438
Weighted average common shares outstanding-basic									18,929	18,673	18,355
Dilutive effect of common stock equivalents									295	622	830
Weighted average common shares outstanding-diluted									19,224	19,295	19,185
Net income per common share-diluted	$ (0.09)	$ (0.06)	$ (0.09)	$ 1.37	$ 0.29	$ 0.13	$ 1.49	$ 2.18	$ 1.13	$ 4.09	$ 1.80
Antidilutive common stock equivalents excluded from above									1,475	605	330

Net Income Per Share - Weighted

Net Income Per Share - Weighted Average Shares Excluded From Calculation of Diluted Net Income Per Share Attributable to Common Stockholders (Detail) - shares	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the computation of diluted net income per share	1,475	605	330
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the computation of diluted net income per share	84,672	25,424
PSU [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Shares excluded from the computation of diluted net income per share	140,210	153,139	157,027

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Commitments And Contingencies [Line Items]
Office lease expiration period	Sep. 30, 2022
Rent expense	$ 2,025,000	$ 1,326,000	$ 948,000
Outstanding letter of credit	608,000	608,000
Restricted cash	608,000	608,000
Tenant improvement allowance	598,000
Company's obligation to pay	5,000,000
Research and Development [Member]
Commitments And Contingencies [Line Items]
Milestone payment made	500,000	0	0
License Agreement Terms [Member]
Commitments And Contingencies [Line Items]
License agreement, recorded liability paid	$ 120,000	$ 200,000	$ 250,000

Commitments and Contingencies59

Commitments and Contingencies - Future Minimum Lease Payments (Detail) $ in Thousands	Sep. 30, 2016USD ($)
Commitments and Contingencies Disclosure [Abstract]
2,017	$ 2,010
2,018	2,062
2,019	2,117
2,020	2,172
2,021	2,229
Thereafter	2,093
Total	12,683
2,017	73
2,018	79
2,019	86
2,020	93
2,021	101
Thereafter	99
Total	$ 531

Income Taxes - Summary of Incom

Income Taxes - Summary of Income Tax (Expense) Benefit (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Current income tax (expense) benefit:
Federal									$ (12,233)	$ (35,040)
State									(956)	(2,172)	$ (151)
Deferred income tax (expense) benefit:
Federal									2,136	(7,371)	13,048
State									159	(1,880)	2,273
Total tax (expense) benefit	$ (826)	$ 434	$ 1,552	$ 9,734	$ (1,629)	$ (428)	$ 20,018	$ 28,502	$ (10,894)	$ (46,463)	$ 15,170

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Tax [Line Items]
Income tax (expense) benefit	$ (826)	$ 434	$ 1,552	$ 9,734	$ (1,629)	$ (428)	$ 20,018	$ 28,502	$ (10,894)	$ (46,463)	$ 15,170
Decreases recorded as benefit to income tax provision											$ 22,892
Effective income tax rate									33.50%	37.00%	(78.70%)
Federal statutory income tax rate									35.00%	35.00%	35.00%
ASU 2015-17 [Member]
Income Tax [Line Items]
Deferred tax assets reclassified as noncurrent	$ 1,700								$ 1,700
Federal [Member]
Income Tax [Line Items]
Net operating loss carryforwards											$ 0
Research and development tax credit carryforwards											4,481
State [Member]
Income Tax [Line Items]
Net operating loss carryforwards											8,433
Research and development tax credit carryforwards											$ 2,214

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Income Tax Provision at Federal Statutory Income Tax Rate and Effective Tax Rate (Detail)	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	35.00%	35.00%	35.00%
State taxes, net of federal benefit	2.70%	2.60%	5.50%
Federal research and development tax credit	(3.80%)	(0.10%)	(0.50%)
Change in deferred tax asset valuation allowance			(118.80%)
Change in statutory rate		0.30%	(2.10%)
Other	(0.40%)	(0.80%)	2.20%
Effective income tax rate	33.50%	37.00%	(78.70%)

Income Taxes - Net Deferred Tax

Income Taxes - Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015
Deferred tax assets:
Capitalized research and development expenses	$ 2,052	$ 2,903
Share-based compensation	4,919	2,129
Other temporary differences	1,498	1,243
Total deferred tax assets	8,469	6,275
Valuation allowance	0	0
Net deferred tax assets	8,469	6,275
Deferred tax liabilities:
Depreciation	(68)	(168)
Unrealized gain (loss)	(11)	(20)
Total deferred tax liabilities	(79)	(188)
Net deferred income tax assets (liabilities)	$ 8,390	$ 6,087

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Uncertain Tax Positions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Beginning Balance	$ 448
Additions based on tax positions for the current period	294	$ 324
Additions based on tax positions for prior periods	16	124
Reductions for tax positions of prior periods	(13)
Ending Balance	$ 745	$ 448

401(k) Plan - Additional Inform

401(k) Plan - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Compensation Related Costs [Abstract]
Benefits charged to operating expenses	$ 513	$ 382	$ 180

Selected Quarterly Financial 66

Selected Quarterly Financial Data (unaudited) - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 12,841	$ 13,978	$ 13,004	$ 48,445	$ 14,416	$ 11,599	$ 57,367	$ 77,498	$ 88,268	$ 160,880	$ 47,741
Operating expenses	15,940	15,067	13,569	12,851	10,742	9,896	8,806	7,288
Other income (expense), net	471	447	472	329	509	287	210	301
Income tax (expense) benefit	826	(434)	(1,552)	(9,734)	1,629	428	(20,018)	(28,502)	10,894	46,463	(15,170)
Net income (loss)	$ (1,802)	$ (1,076)	$ (1,645)	$ 26,189	$ 5,812	$ 2,418	$ 28,753	$ 42,009	$ 21,666	$ 78,992	$ 34,438
Net income (loss) per common share-basic	$ (0.09)	$ (0.06)	$ (0.09)	$ 1.39	$ 0.30	$ 0.13	$ 1.54	$ 2.26	$ 1.14	$ 4.23	$ 1.88
Net income (loss) per common share-diluted	$ (0.09)	$ (0.06)	$ (0.09)	$ 1.37	$ 0.29	$ 0.13	$ 1.49	$ 2.18	$ 1.13	$ 4.09	$ 1.80

Selected Quarterly Financial 67

Selected Quarterly Financial Data (unaudited) - Schedule of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Schedule Of Quarterly Financial Information [Line Items]
Milestones				$ 30,000	$ 125,000	$ 40,000
AbbVie [Member] | Japan [Member]
Schedule Of Quarterly Financial Information [Line Items]
Milestones	$ 30,000
AbbVie [Member] | United States [Member]
Schedule Of Quarterly Financial Information [Line Items]
Milestones			$ 75,000
AbbVie [Member] | Europe [Member]
Schedule Of Quarterly Financial Information [Line Items]
Milestones		$ 50,000