Enanta Pharmaceuticals (ENTA) 10-Q 10 May 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Mar. 31, 2019	Apr. 28, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Trading Symbol	ENTA
Entity Registrant Name	ENANTA PHARMACEUTICALS INC
Entity Central Index Key	0001177648
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		19,669,726

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Current assets:
Cash and cash equivalents	$ 106,030	$ 63,902
Short-term marketable securities	264,127	244,828
Accounts receivable	39,631	67,205
Prepaid expenses and other current assets	14,052	4,454
Total current assets	423,840	380,389
Long-term marketable securities	16,510	16,389
Property and equipment, net	11,476	8,374
Deferred tax assets	9,052	8,375
Restricted cash	608	608
Other long-term assets	92	92
Total assets	461,578	414,227
Current liabilities:
Accounts payable	8,884	4,745
Accrued expenses and other current liabilities	11,607	9,892
Income taxes payable		1,388
Total current liabilities	20,491	16,025
Series 1 nonconvertible preferred stock	1,628	1,628
Other long-term liabilities	3,249	2,895
Total liabilities	25,368	20,548
Commitments and contingencies (Note 11)
Stockholders' equity:
Common stock; $0.01 par value per share, 100,000 shares authorized; 19,669 and 19,395 shares issued and outstanding at March 31, 2019 and September 30, 2018, respectively	197	194
Additional paid-in capital	288,512	276,526
Accumulated other comprehensive loss	(12)	(398)
Retained earnings	147,513	117,357
Total stockholders' equity	436,210	393,679
Total liabilities and stockholders' equity	$ 461,578	$ 414,227

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Sep. 30, 2018
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	19,669,000	19,395,000
Common stock, shares outstanding	19,669,000	19,395,000

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Revenue
Total revenue	$ 39,631	$ 44,049	$ 109,517	$ 82,158
Operating expenses:
Research and development	34,155	21,484	69,033	39,446
General and administrative	6,780	5,706	13,932	11,476
Total operating expenses	40,935	27,190	82,965	50,922
Income (loss) from operations	(1,304)	16,859	26,552	31,236
Other income, net	2,245	1,066	4,130	2,026
Income before income taxes	941	17,925	30,682	33,262
Income tax (expense) benefit	3,204	(5,370)	(526)	(9,014)
Net income	$ 4,145	$ 12,555	$ 30,156	$ 24,248
Net income per share:
Basic	$ 0.21	$ 0.65	$ 1.55	$ 1.27
Diluted	$ 0.20	$ 0.61	$ 1.44	$ 1.20
Weighted average shares outstanding:
Basic	19,549	19,206	19,487	19,167
Diluted	21,084	20,601	20,946	20,256
Royalties [Member]
Revenue
Total revenue	$ 39,631	$ 44,049	$ 109,517	$ 67,158
Milestone [Member]
Revenue
Total revenue				$ 15,000

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 4,145	$ 12,555	$ 30,156	$ 24,248
Other comprehensive income (loss):
Net unrealized gains (losses) on marketable securities, net of tax of $78, ($128), $123 and ($235)	245	(336)	386	(682)
Total other comprehensive income (loss)	245	(336)	386	(682)
Comprehensive income	$ 4,390	$ 12,219	$ 30,542	$ 23,566

Consolidated Statements of Co_2

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net unrealized gains (losses) on marketable securities, tax	$ 78	$ (128)	$ 123	$ (235)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Beginning Balance at Sep. 30, 2017	$ 301,676	$ 191	$ 256,241	$ (112)	$ 45,356
Beginning Balance, Shares at Sep. 30, 2017		19,120
Exercise of stock options and warrants	436		436
Exercise of stock options and warrants, Shares		30
Stock-based compensation expense	4,120		4,120
Other comprehensive income (loss)	(346)			(346)
Net income	11,693				11,693
Ending Balance at Dec. 31, 2017	317,579	$ 191	260,752	(458)	57,094
Ending Balance, Shares at Dec. 31, 2017		19,150
Beginning Balance at Sep. 30, 2017	301,676	$ 191	256,241	(112)	45,356
Beginning Balance, Shares at Sep. 30, 2017		19,120
Other comprehensive income (loss)	(682)
Net income	24,248
Ending Balance at Mar. 31, 2018	333,402	$ 193	264,354	(794)	69,649
Ending Balance, Shares at Mar. 31, 2018		19,260
Cumulative effect adjustment for adoption of new accounting guidance			(45)		45
Beginning Balance at Dec. 31, 2017	317,579	$ 191	260,752	(458)	57,094
Beginning Balance, Shares at Dec. 31, 2017		19,150
Exercise of stock options	1,770	$ 1	1,769
Exercise of stock options, Shares		65
Vesting of restricted stock units, net of withholding	(1,756)	$ 1	(1,757)
Vesting of restricted stock units, net of withholding, Shares		45
Stock-based compensation expense	3,590		3,590
Other comprehensive income (loss)	(336)			(336)
Net income	12,555				12,555
Ending Balance at Mar. 31, 2018	333,402	$ 193	264,354	(794)	69,649
Ending Balance, Shares at Mar. 31, 2018		19,260
Beginning Balance at Sep. 30, 2018	393,679	$ 194	276,526	(398)	117,357
Beginning Balance, Shares at Sep. 30, 2018		19,395
Exercise of stock options	1,161		1,161
Exercise of stock options, Shares		40
Stock-based compensation expense	5,843		5,843
Other comprehensive income (loss)	141			141
Net income	26,011				26,011
Ending Balance at Dec. 31, 2018	426,835	$ 194	283,530	(257)	143,368
Ending Balance, Shares at Dec. 31, 2018		19,435
Beginning Balance at Sep. 30, 2018	$ 393,679	$ 194	276,526	(398)	117,357
Beginning Balance, Shares at Sep. 30, 2018		19,395
Exercise of stock options, Shares	197
Vesting of restricted stock units, net of withholding, Shares		77
Other comprehensive income (loss)	$ 386
Net income	30,156
Ending Balance at Mar. 31, 2019	436,210	$ 197	288,512	(12)	147,513
Ending Balance, Shares at Mar. 31, 2019		19,669
Beginning Balance at Dec. 31, 2018	426,835	$ 194	283,530	(257)	143,368
Beginning Balance, Shares at Dec. 31, 2018		19,435
Exercise of stock options	4,466	$ 2	4,464
Exercise of stock options, Shares		157
Vesting of restricted stock units, net of withholding	(4,188)	$ 1	(4,189)
Vesting of restricted stock units, net of withholding, Shares		77
Stock-based compensation expense	4,707		4,707
Other comprehensive income (loss)	245			245
Net income	4,145				4,145
Ending Balance at Mar. 31, 2019	$ 436,210	$ 197	$ 288,512	$ (12)	$ 147,513
Ending Balance, Shares at Mar. 31, 2019		19,669

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities
Net income	$ 30,156	$ 24,248
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	10,550	7,710
Depreciation and amortization expense	1,482	1,200
Deferred income taxes	(800)	2,791
Premium paid on marketable securities	(1)	(243)
Amortization of (accretion of) premium (discount) on marketable securities	(2,213)	110
Change in fair value of warrant liability and Series 1 nonconvertible preferred stock		(41)
Other non-cash items	37	(84)
Change in operating assets and liabilities:
Accounts receivable	27,574	(33,435)
Prepaid expenses and other current assets	(9,598)	(1,369)
Accounts payable	2,886	565
Accrued expenses	1,259	(939)
Income taxes payable	(1,388)	(3,918)
Other long-term liabilities	396	236
Net cash provided by (used in) operating activities	60,340	(3,169)
Cash flows from investing activities
Purchase of marketable securities	(271,712)	(99,721)
Proceeds from maturities and sale of marketable securities	254,994	104,986
Purchase of property and equipment	(2,891)	(1,256)
Net cash provided by (used in) investing activities	(19,609)	4,009
Cash flows from financing activities
Proceeds from exercise of stock options and warrants	5,627	2,207
Payments for settlement of share-based awards	(4,188)	(1,757)
Payments of capital lease obligations	(42)	(39)
Net cash provided by financing activities	1,397	411
Net increase in cash, cash equivalents and restricted cash	42,128	1,251
Cash, cash equivalents and restricted cash at beginning of period	64,510	66,283
Cash, cash equivalents and restricted cash at end of period	106,638	67,534
Supplemental disclosure of cash flow information:
Cash paid for income taxes	9,955	9,883
Supplemental disclosure of non-cash investing information:
Purchases of fixed assets included in accounts payable and accrued expenses	$ 1,598	$ 111

Nature of the Business and Basi

Nature of the Business and Basis of Presentation	6 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Enanta Pharmaceuticals, Inc. (the “Company”), incorporated in Delaware in 1995, is a 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 discovered glecaprevir, the second protease inhibitor discovered and developed through its collaboration with AbbVie, which is marketed as part of AbbVie’s direct-acting antiviral (DAA) regimen under the tradenames MAVYRET™ (U.S.) or MAVIRET™ (ex-U.S.) (glecaprevir/pibrentasvir) for the treatment of chronic hepatitis C virus, or HCV. The other protease inhibitor under its HCV collaboration, which is part of AbbVie’s initial DAA regimens for the treatment of chronic HCV, is currently marketed outside the U.S. under the tradename VIEKIRAX ® (paritaprevir/ritonavir/ombitasvir). , which are currently focused on the following disease targets The Company is subject to many of the 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 and compliance with government regulation. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approvals, prior to commercialization. These efforts require significant amounts of capital, adequate personnel infrastructure, and extensive compliance reporting capabilities. Unaudited Interim Financial Information The consolidated balance sheet at September 30, 2018 was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America (“GAAP”). The accompanying unaudited consolidated financial statements as of March 31, 2019 and for the three and six months ended March 31, 2019 and 2018 have been prepared by the Company pursuant to the rules and regulations of the Securities and Exchange Commission (“SEC”) for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to such rules and regulations. These financial statements should be read in conjunction with the Company’s audited financial statements and the notes thereto included in the Company’s Annual Report on Form 10-K for the year ended September 30, 2018. In the opinion of management, all adjustments, consisting of normal recurring adjustments necessary for a fair statement of the Company’s financial position as of March 31, 2019 and results of operations for the three and six months ended March 31, 2019 and 2018 and cash flows for the six months ended March 31, 2019 and 2018, have been made. The results of operations for the six months ended March 31, 2019 are not necessarily indicative of the results of operations that may be expected for subsequent quarters or the year ending September 30, 2019. The accompanying consolidated financial statements have been prepared in conformity with GAAP. All dollar amounts in the consolidated financial statements and in the notes to the consolidated financial statements, except per share amounts, are in thousands unless otherwise indicated.

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies For the Company’s Significant Accounting Policies, please refer to its Annual Report on Form 10-K for the fiscal year ended September 30, 2018. Other than the adoption of ASC 606 as of October 1, 2018, there were no other significant changes to the Company’s Significant Accounting Policies during the quarter. 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 with respect to its revenue arrangements; valuation of stock-based awards; the accrual of research and development expenses, and the accounting for income taxes, including uncertain tax positions and the valuation of net deferred tax assets. Revenue Recognition On October 1, 2018, the Company adopted the new revenue standard, discussed below, which amended revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries (“ASC 606”). ASC 606 provides guidance on recognizing revenue, including a five-step model to determine when revenue recognition is appropriate. The standard requires that an entity recognize revenue to depict the transfer of control of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers and has since issued several additional amendments thereto, collectively referred to herein as ASC 606 Multiple-Element Arrangements one In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”) that changes the presentation of restricted cash and cash equivalents on the statement of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This amendment was effective for the Company in the fiscal year beginning October 1, 2018. The Company adopted ASU 2016-18 retrospectively as of October 1, 2018. $608 In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) (“ASU 2017-09”) which In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which will replace the existing guidance in ASC 840, “Leases.” The updated standard aims to increase transparency and comparability among organizations by requiring lessees to recognize leased assets and leased liabilities on the consolidated balance sheets and requiring disclosure of key information about leasing arrangements. This amendment is effective for the Company in the fiscal year beginning October 1, 2019, but early adoption is permissible. The Company is currently evaluating the potential impact that ASU 2016-02 may have on its financial position and results of operations. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) (“ASU 2016-13”), which introduces a new methodology for accounting for credit losses on financial instruments, including available-for-sale debt securities. The guidance establishes a new “expected loss model” that requires entities to estimate current expected credit losses on financial instruments by using all practical and relevant information. Any expected credit losses are to be reflected as allowances rather than reductions in the amortized cost of available-for-sale debt securities. This amendment is effective for the Company in the fiscal year beginning October 1, 2020. The Company is currently evaluating the potential impact that ASU 2016-13 may have on its financial position and results of operations. In March 2017, the FASB issued ASU No. 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities (“ASU 2017-08”) which requires companies to amend the amortization period for premiums on debt securities with explicit call features to be the earliest call date rather than through the contractual life of the debt instrument. This amendment aims to more closely align the recognition of interest income with the manner in which market participants price such instruments. This amendment is effective for the Company in the fiscal year beginning October 1, 2019, but early adoption is permissible. The Company is currently evaluating the potential impact that ASU 2017-08 may have on its financial position and results of operations. 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	6 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and September 30, 2018 and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value: Fair Value Measurements at March 31, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 89,172 $ — $ — $ 89,172 Commercial paper — 10,954 — 10,954 Marketable securities: U.S. Treasury notes 106,670 — — 106,670 Commercial paper — 100,829 — 100,829 Corporate bonds — 73,138 — 73,138 $ 195,842 $ 184,921 $ — $ 380,763 Liabilities: Series 1 nonconvertible preferred stock — — 1,628 1,628 $ — $ — $ 1,628 $ 1,628 Fair Value Measurements at September 30, 2018 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 51,025 $ — $ — $ 51,025 Commercial paper — 6,987 — 6,987 Corporate bonds — 3,998 — 3,998 Marketable securities: U.S. Treasury notes 42,703 — — 42,703 Commercial paper — 113,885 — 113,885 Corporate bonds — 104,629 — 104,629 $ 93,728 $ 229,499 $ — $ 323,227 Liabilities: Series 1 nonconvertible preferred stock — — 1,628 1,628 $ — $ — $ 1,628 $ 1,628 During the six months ended March 31, 2019 and 2018, there were no transfers between Level 1, Level 2 and Level 3. The outstanding shares of Series 1 nonconvertible preferred stock are measured at fair value. The fair value of these instruments was 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 value of the warrant liability (for the period the warrants were outstanding) 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 outstanding Series 1 nonconvertible preferred stock using probability-weighted discounted cash flow include the following significant unobservable inputs: Range (Weighted Average) March 31, September 30, Unobservable Input 2019 2018 Probabilities of payout 0%-70% 0%-70% Discount rate 6.25% 6.25% The following table provides a rollforward of the aggregate fair values of the Company’s outstanding Series 1 nonconvertible preferred stock for which fair value is determined by Level 3 inputs: Series 1 Nonconvertible Preferred Stock Balance, September 30, 2018 $ 1,628 Change in fair value of nonconvertible preferred stock — Balance, March 31, 2019 $ 1,628

Marketable Securities

Marketable Securities	6 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Marketable Securities	4. Marketable Securities As of March 31, 2019 and September 30, 2018, the fair value of available-for-sale marketable securities, by type of security, was as follows: March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) U.S. Treasury notes $ 106,648 $ 29 $ (7 ) 106,670 Commercial paper 100,829 — — 100,829 Corporate bonds 73,225 49 (136 ) 73,138 $ 280,702 $ 78 $ (143 ) $ 280,637 September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Commercial paper $ 113,885 $ — $ — $ 113,885 Corporate bonds 105,105 1 (477 ) 104,629 U.S. Treasury notes 42,801 — (98 ) 42,703 $ 261,791 $ 1 $ (575 ) $ 261,217 As of March 31, 2019, marketable securities consisted of short-term marketable securities, which are investments that mature within one year, and long-term marketable securities, with an aggregate fair value of $16,510, which consist of certain corporate bonds that have maturities of more than one year but not more than three years.

Accrued Expenses and Other Long

Accrued Expenses and Other Long-Term Liabilities	6 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Long-Term Liabilities	5. Accrued Expenses and Other Long-Term Liabilities Accrued expenses and other current liabilities as well as other long-term liabilities consisted of the following as of March 31, 2019 and September 30, 2018: March 31, September 2019 2018 (in thousands) Accrued expenses: Accrued preclinical and clinical expenses $ 5,530 $ 3,617 Accrued payroll and related expenses 1,720 3,274 Accrued vendor manufacturing 2,695 1,901 Accrued professional fees 742 507 Accrued other 920 593 $ 11,607 $ 9,892 Other long-term liabilities: Uncertain tax positions $ 1,922 $ 1,792 Accrued rent expense 842 593 Capital lease obligation 249 293 Asset retirement obligation 236 217 $ 3,249 $ 2,895

Ongoing Collaboration Agreement

Ongoing Collaboration Agreements	6 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Ongoing Collaboration Agreements	6. Ongoing Collaboration Agreements AbbVie Collaboration The Company has a Collaborative Development and License Agreement (as amended, the “AbbVie Agreement”), with AbbVie to identify, develop and commercialize HCV NS3 and NS3/4A protease inhibitor compounds, including paritaprevir and glecaprevir, under which the Company has received license payments, proceeds from a sale of preferred stock, research funding payments, milestone payments and royalties totaling approximately $787,000 through March 31, 2019. Since the Company satisfied all of its performance obligations under the AbbVie Agreement by the end of fiscal 2011, all milestone payments received since then have been recognized as revenue when the milestones were achieved by AbbVie. During the six months ended March 31, 2018, the Company earned and recognized milestone revenue of $15,000 upon AbbVie’s achievement of commercialization regulatory approval in Japan for MAVIRET™, the final milestone payment under this agreement The Company is also receiving annually tiered royalties per Company protease inhibitor product ranging from ten percent up to twenty percent, or on a blended basis from ten percent up to the high teens, on the portion of AbbVie’s calendar year net sales of each HCV regimen that is allocated to the protease inhibitor in the regimen. Beginning with each January 1, the cumulative net sales of a given royalty-bearing protease inhibitor product start at zero for purposes of calculating the tiered royalties on a product-by-product basis.

Series 1 Nonconvertible Preferr

Series 1 Nonconvertible Preferred Stock	6 Months Ended
	Mar. 31, 2019
Debt Disclosure [Abstract]
Series 1 Nonconvertible Preferred Stock	7. Series 1 Nonconvertible Preferred Stock As of March 31, 2019, 1,931 shares of Series 1 nonconvertible preferred stock were issued and outstanding. Since these shares qualify as a derivative, the outstanding shares are carried at fair value as a liability on the Company’s consolidated balance sheet.

Stock-Based Awards

Stock-Based Awards	6 Months Ended
	Mar. 31, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	8. Stock-Based Awards The Company grants stock-based awards, including stock options and unit awards under its 2019 Equity Incentive Plan (the “2019 Plan”), which was approved by our stockholders on February 28, 2019. The Company also has outstanding unit awards, stock options and restricted stock unit awards under its 2012 Equity Incentive Plan (the “2012 Plan”) and its amended and restated 1995 Equity Incentive Plan (the “1995 Plan”), but is no longer granting awards under these plans. The following table summarizes stock option activity, including performance-based options, for the year-to-date period ending March 31, 2019: Shares Issuable Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term in years Aggregate Intrinsic Value (in thousands) (in thousands) Outstanding as of September 30, 2018 2,624 $ 36.65 7.1 $ 129,115 Granted 537 82.21 Exercised (197 ) 28.59 Forfeited (32 ) 70.59 Outstanding as of March 31, 2019 2,932 $ 45.17 7.2 $ 148,329 Options vested and expected to vest as of March 31, 2019 2,932 $ 45.17 7.2 $ 148,329 Options exercisable as of March 31, 2019 1,722 $ 34.47 6.1 $ 105,171 Market and Performance-Based Stock Unit Awards The Company awards both performance share units, or PSUs, and relative total stockholder 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. PSUs rTSRUs Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2018 70 $ 50.97 70 $ 59.96 Granted 21 67.13 21 47.42 Vested (36 ) 35.89 (45 ) 46.11 Cancelled (9 ) 35.89 — — Unvested at March 31, 2019 46 $ 73.03 46 $ 67.80 During the six months ended March 31, 2019, a total of 80% of the target PSUs and 200% of the target rTSRUs granted in January 2017 vested, resulting in the issuance of an aggregate of 77 common shares, net of share withholding. Restricted Stock Units During the three months ended December 31, 2016, the Company awarded restricted stock units to its employees, which vest 50% in three years and 50% in four years, provided the employee remains employed with the Company at the time of vesting. The fair value per share of these awards is determined based on the intrinsic value of the stock on the date of grant and will be recognized as stock-based compensation expense over the requisite service period. The following table summarizes the restricted stock unit activity for the year-to-date period ending March 31, 2019: Restricted Stock Units Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2018 109 $ 30.00 Granted — — Vested — — Cancelled (1 ) 30.00 Unvested at March 31, 2019 108 $ 30.00 Stock-Based Compensation Expense During the six months ended March 31, 2019 and 2018, the Company recognized the following stock-based compensation expense: Three Months ended Six Months Ended March 31, March 31, 2019 2018 2019 2018 (in thousands) Research and development $ 2,194 $ 1,429 $ 4,468 $ 2,900 General and administrative 2,513 2,161 6,082 4,810 $ 4,707 $ 3,590 $ 10,550 $ 7,710 Three Months ended Six Months Ended March 31, March 31, 2019 2018 2019 2018 (in thousands) Stock options $ 3,949 $ 3,030 $ 7,853 $ 5,978 Performance stock units 72 35 1,278 641 rTSRUs 493 332 1,025 694 Restricted stock units 193 193 394 397 $ 4,707 $ 3,590 $ 10,550 $ 7,710 During the six months ended March 31, 2019 and 2018, the Company recognized stock-based compensation expense for PSUs and performance-based options upon achievement of performance-based targets that occurred during their respective periods. As of March 31, 2019, the Company had an aggregate of $44,379 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.6 years.

Net Income Per Share

Net Income Per Share	6 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Income Per Share	9. Net Income Per Share Basic and diluted net income per share attributable to common stockholders was calculated as follows for six months ended March 31, 2019 and 2018 (in thousands, except per share data): Three Months Ended Six Months Ended March 31, March 31, 2019 2018 2019 2018 (in thousands, except per share data) Basic net income per share: Numerator: Net income $ 4,145 $ 12,555 $ 30,156 $ 24,248 Denominator: Weighted average common shares outstanding—basic 19,549 19,206 19,487 19,167 Net income per share common share—basic $ 0.21 $ 0.65 $ 1.55 $ 1.27 Diluted net income per share: Numerator: Net income $ 4,145 $ 12,555 $ 30,156 $ 24,248 Denominator: Weighted average common shares outstanding—basic 19,549 19,206 19,487 19,167 Dilutive effect of common stock equivalents 1,535 1,395 1,459 1,089 Weighted average common shares outstanding—diluted 21,084 20,601 20,946 20,256 Net income per share common share—diluted $ 0.20 $ 0.61 $ 1.44 $ 1.20 Anti-dilutive common stock equivalents excluded from above 784 46 694 449

Income Taxes

Income Taxes	6 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes For the three months ended March 31, 2019 and 2018, the Company recorded an income tax benefit of $3,204 and income tax (expense) of ($5,370) respectively, both of which were attributable to the Company’s domestic operations. During the three months ended March 31, 2019, the Company recorded an income tax benefit, despite its pre-tax income, due to tax deductions for employee stock award-related activity during the quarter. representing effective tax rates of 1.7% 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. The Company’s tax years are still open under statute from 2014 to the present. Earlier years may be examined to the extent that tax credit or net operating loss carryforwards are used in future periods. During 2018, the Company received notice of examination by the Internal Revenue Service (“IRS”) for the year ending September 30, 2016. The Company received and agreed to a notice of proposed adjustment from the IRS which was paid in September 2018, the amount of which was immaterial to the financial statements. The Company is in the process of finalizing the completion of the IRS audit. During October 2018, the Company received notice of examination by the Massachusetts Department of Revenue (“DOR”) for the years ending September 30, 2015 and September 30, 2016. No adjustments have been proposed to date. The Company has not received notice of examination by any other jurisdictions for any other tax year open under statute. The Company had an unrecognized tax benefit of $1,922 and $1,792 as of March 31, 2019 and September 30, 2018, respectively. Unrecognized tax benefits 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.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Mar. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Leases The Company has two leases for office and laboratory space located in Watertown, Massachusetts that expire in September 2022 and August 2024 The Company recorded rent expense of $1,246 and $1,013 for the six months ended March 31, 2019 and 2018, respectively. In connection with one of the leases, the Company has an outstanding letter of credit in the amount of $608 as of March 31, 2019 and September 30, 2018, collateralized by a money market account. As of March 31, 2019 and September 30, 2018, the Company classified the $608 related to the money market account as long-term restricted cash. 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 could 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 or from services to be provided to 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 and its executive officers 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 March 31, 2019.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
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 with respect to its revenue arrangements; valuation of stock-based awards; the accrual of research and development expenses, and the accounting for income taxes, including uncertain tax positions and the valuation of net deferred tax assets.
Revenue Recognition	Revenue Recognition On October 1, 2018, the Company adopted the new revenue standard, discussed below, which amended revenue recognition principles and provides a single, comprehensive set of criteria for revenue recognition within and across all industries (“ASC 606”). ASC 606 provides guidance on recognizing revenue, including a five-step model to determine when revenue recognition is appropriate. The standard requires that an entity recognize revenue to depict the transfer of control of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers and has since issued several additional amendments thereto, collectively referred to herein as ASC 606 Multiple-Element Arrangements one In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”) that changes the presentation of restricted cash and cash equivalents on the statement of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. This amendment was effective for the Company in the fiscal year beginning October 1, 2018. The Company adopted ASU 2016-18 retrospectively as of October 1, 2018. $608 In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) (“ASU 2017-09”) which In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) (“ASU 2016-02”), which will replace the existing guidance in ASC 840, “Leases.” The updated standard aims to increase transparency and comparability among organizations by requiring lessees to recognize leased assets and leased liabilities on the consolidated balance sheets and requiring disclosure of key information about leasing arrangements. This amendment is effective for the Company in the fiscal year beginning October 1, 2019, but early adoption is permissible. The Company is currently evaluating the potential impact that ASU 2016-02 may have on its financial position and results of operations. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326) (“ASU 2016-13”), which introduces a new methodology for accounting for credit losses on financial instruments, including available-for-sale debt securities. The guidance establishes a new “expected loss model” that requires entities to estimate current expected credit losses on financial instruments by using all practical and relevant information. Any expected credit losses are to be reflected as allowances rather than reductions in the amortized cost of available-for-sale debt securities. This amendment is effective for the Company in the fiscal year beginning October 1, 2020. The Company is currently evaluating the potential impact that ASU 2016-13 may have on its financial position and results of operations. In March 2017, the FASB issued ASU No. 2017-08, Receivables—Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities (“ASU 2017-08”) which requires companies to amend the amortization period for premiums on debt securities with explicit call features to be the earliest call date rather than through the contractual life of the debt instrument. This amendment aims to more closely align the recognition of interest income with the manner in which market participants price such instruments. This amendment is effective for the Company in the fiscal year beginning October 1, 2019, but early adoption is permissible. The Company is currently evaluating the potential impact that ASU 2017-08 may have on its financial position and results of operations. 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 Asset_2

Fair Value of Financial Assets and Liabilities (Tables)	6 Months Ended
	Mar. 31, 2019
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 March 31, 2019 and September 30, 2018 and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value: Fair Value Measurements at March 31, 2019 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 89,172 $ — $ — $ 89,172 Commercial paper — 10,954 — 10,954 Marketable securities: U.S. Treasury notes 106,670 — — 106,670 Commercial paper — 100,829 — 100,829 Corporate bonds — 73,138 — 73,138 $ 195,842 $ 184,921 $ — $ 380,763 Liabilities: Series 1 nonconvertible preferred stock — — 1,628 1,628 $ — $ — $ 1,628 $ 1,628 Fair Value Measurements at September 30, 2018 Using: Level 1 Level 2 Level 3 Total (in thousands) Assets: Cash equivalents: Money market funds $ 51,025 $ — $ — $ 51,025 Commercial paper — 6,987 — 6,987 Corporate bonds — 3,998 — 3,998 Marketable securities: U.S. Treasury notes 42,703 — — 42,703 Commercial paper — 113,885 — 113,885 Corporate bonds — 104,629 — 104,629 $ 93,728 $ 229,499 $ — $ 323,227 Liabilities: Series 1 nonconvertible preferred stock — — 1,628 1,628 $ — $ — $ 1,628 $ 1,628
Fair Value Measurements of the Company's Outstanding Series 1 Nonconvertible Preferred Stock	The recurring Level 3 fair value measurements of the Company’s outstanding Series 1 nonconvertible preferred stock using probability-weighted discounted cash flow include the following significant unobservable inputs: Range (Weighted Average) March 31, September 30, Unobservable Input 2019 2018 Probabilities of payout 0%-70% 0%-70% Discount rate 6.25% 6.25%
Rollforward of Aggregate Fair Values of Outstanding Series 1 Nonconvertible Preferred Stock	The following table provides a rollforward of the aggregate fair values of the Company’s outstanding Series 1 nonconvertible preferred stock for which fair value is determined by Level 3 inputs: Series 1 Nonconvertible Preferred Stock Balance, September 30, 2018 $ 1,628 Change in fair value of nonconvertible preferred stock — Balance, March 31, 2019 $ 1,628

Marketable Securities (Tables)

Marketable Securities (Tables)	6 Months Ended
	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Fair Value of Available-for-Sale Marketable Securities by Type of Security	As of March 31, 2019 and September 30, 2018, the fair value of available-for-sale marketable securities, by type of security, was as follows: March 31, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) U.S. Treasury notes $ 106,648 $ 29 $ (7 ) 106,670 Commercial paper 100,829 — — 100,829 Corporate bonds 73,225 49 (136 ) 73,138 $ 280,702 $ 78 $ (143 ) $ 280,637 September 30, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Commercial paper $ 113,885 $ — $ — $ 113,885 Corporate bonds 105,105 1 (477 ) 104,629 U.S. Treasury notes 42,801 — (98 ) 42,703 $ 261,791 $ 1 $ (575 ) $ 261,217

Accrued Expenses and Other Lo_2

Accrued Expenses and Other Long-Term Liabilities (Tables)	6 Months Ended
	Mar. 31, 2019
Payables And Accruals [Abstract]
Accrued Expenses and Other Current Liabilities as well as Other Long-Term Liabilities	Accrued expenses and other current liabilities as well as other long-term liabilities consisted of the following as of March 31, 2019 and September 30, 2018: March 31, September 2019 2018 (in thousands) Accrued expenses: Accrued preclinical and clinical expenses $ 5,530 $ 3,617 Accrued payroll and related expenses 1,720 3,274 Accrued vendor manufacturing 2,695 1,901 Accrued professional fees 742 507 Accrued other 920 593 $ 11,607 $ 9,892 Other long-term liabilities: Uncertain tax positions $ 1,922 $ 1,792 Accrued rent expense 842 593 Capital lease obligation 249 293 Asset retirement obligation 236 217 $ 3,249 $ 2,895

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	6 Months Ended
	Mar. 31, 2019
Summary of Stock Option Activity Including Performance Based Options	The following table summarizes stock option activity, including performance-based options, for the year-to-date period ending March 31, 2019: Shares Issuable Under Options Weighted Average Exercise Price Weighted Average Remaining Contractual Term in years Aggregate Intrinsic Value (in thousands) (in thousands) Outstanding as of September 30, 2018 2,624 $ 36.65 7.1 $ 129,115 Granted 537 82.21 Exercised (197 ) 28.59 Forfeited (32 ) 70.59 Outstanding as of March 31, 2019 2,932 $ 45.17 7.2 $ 148,329 Options vested and expected to vest as of March 31, 2019 2,932 $ 45.17 7.2 $ 148,329 Options exercisable as of March 31, 2019 1,722 $ 34.47 6.1 $ 105,171
Summary of Restricted Stock Unit Activity	The following table summarizes the restricted stock unit activity for the year-to-date period ending March 31, 2019: Restricted Stock Units Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2018 109 $ 30.00 Granted — — Vested — — Cancelled (1 ) 30.00 Unvested at March 31, 2019 108 $ 30.00
Stock-Based Compensation Expense	During the six months ended March 31, 2019 and 2018, the Company recognized the following stock-based compensation expense: Three Months ended Six Months Ended March 31, March 31, 2019 2018 2019 2018 (in thousands) Research and development $ 2,194 $ 1,429 $ 4,468 $ 2,900 General and administrative 2,513 2,161 6,082 4,810 $ 4,707 $ 3,590 $ 10,550 $ 7,710 Three Months ended Six Months Ended March 31, March 31, 2019 2018 2019 2018 (in thousands) Stock options $ 3,949 $ 3,030 $ 7,853 $ 5,978 Performance stock units 72 35 1,278 641 rTSRUs 493 332 1,025 694 Restricted stock units 193 193 394 397 $ 4,707 $ 3,590 $ 10,550 $ 7,710
Performance Share Units and Relative Total Stockholder Return Units [Member]
Summary of PSUs and rTSRUs Activity Based on Targeted Number of Shares	The following table summarizes PSU and rTSRU activity based on the targeted number of shares for the year-to-date period ending March 31, 2019: PSUs rTSRUs Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value (in thousands, except per share data) Unvested at September 30, 2018 70 $ 50.97 70 $ 59.96 Granted 21 67.13 21 47.42 Vested (36 ) 35.89 (45 ) 46.11 Cancelled (9 ) 35.89 — — Unvested at March 31, 2019 46 $ 73.03 46 $ 67.80

Net Income Per Share (Tables)

Net Income Per Share (Tables)	6 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Basic and Diluted Net Income Per Share Attributable to Common Stockholders	Basic and diluted net income per share attributable to common stockholders was calculated as follows for six months ended March 31, 2019 and 2018 (in thousands, except per share data): Three Months Ended Six Months Ended March 31, March 31, 2019 2018 2019 2018 (in thousands, except per share data) Basic net income per share: Numerator: Net income $ 4,145 $ 12,555 $ 30,156 $ 24,248 Denominator: Weighted average common shares outstanding—basic 19,549 19,206 19,487 19,167 Net income per share common share—basic $ 0.21 $ 0.65 $ 1.55 $ 1.27 Diluted net income per share: Numerator: Net income $ 4,145 $ 12,555 $ 30,156 $ 24,248 Denominator: Weighted average common shares outstanding—basic 19,549 19,206 19,487 19,167 Dilutive effect of common stock equivalents 1,535 1,395 1,459 1,089 Weighted average common shares outstanding—diluted 21,084 20,601 20,946 20,256 Net income per share common share—diluted $ 0.20 $ 0.61 $ 1.44 $ 1.20 Anti-dilutive common stock equivalents excluded from above 784 46 694 449

Nature of the Business and Ba_2

Nature of the Business and Basis of Presentation - Additional Information (Detail)	6 Months Ended
	Mar. 31, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Entity incorporated, in year	1995
Entity incorporation, state country name	Delaware

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018	Dec. 31, 2017
Summary Of Accounting And Financial Policies [Line Items]
Restricted cash	$ 608	$ 608
ASU 2016-18 [Member]
Summary Of Accounting And Financial Policies [Line Items]
Restricted cash			$ 608

Fair Value of Financial Asset_3

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	Mar. 31, 2019	Sep. 30, 2018
Assets:
Marketable securities	$ 280,637	$ 261,217
Assets, Fair Value Disclosure, Total	380,763	323,227
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,628	1,628
Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,628	1,628
Level 1 [Member]
Assets:
Assets, Fair Value Disclosure, Total	195,842	93,728
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	184,921	229,499
Level 3 [Member]
Liabilities:
Liabilities, Fair Value Disclosure, Total	1,628	1,628
Level 3 [Member] | Series 1 Nonconvertible Preferred Stock [Member]
Liabilities:
Liabilities	1,628	1,628
Money Market Funds [Member]
Assets:
Cash equivalents	89,172	51,025
Money Market Funds [Member] | Level 1 [Member]
Assets:
Cash equivalents	89,172	51,025
Corporate Bonds [Member]
Assets:
Cash equivalents		3,998
Marketable securities	73,138	104,629
Corporate Bonds [Member] | Level 2 [Member]
Assets:
Cash equivalents		3,998
Marketable securities	73,138	104,629
Commercial Paper [Member]
Assets:
Cash equivalents	10,954	6,987
Marketable securities	100,829	113,885
Commercial Paper [Member] | Level 2 [Member]
Assets:
Cash equivalents	10,954	6,987
Marketable securities	100,829	113,885
U.S. Treasury Notes [Member]
Assets:
Marketable securities	106,670	42,703
U.S. Treasury Notes [Member] | Level 1 [Member]
Assets:
Marketable securities	$ 106,670	$ 42,703

Fair Value of Financial Asset_4

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0

Fair Value of Financial Asset_5

Fair Value of Financial Assets and Liabilities - Fair Value Measurements of the Company's Outstanding Series 1 Nonconvertible Preferred Stock (Detail) - Level 3 [Member]	Mar. 31, 2019	Sep. 30, 2018
Discount Rate [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Range (Weighted Average)	0.0625	0.0625
Minimum [Member] | Probabilities of Payout [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Range (Weighted Average)	0	0
Maximum [Member] | Probabilities of Payout [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Range (Weighted Average)	0.70	0.70

Fair Value of Financial Asset_6

Fair Value of Financial Assets and Liabilities - Rollforward of Aggregate Fair Values of Outstanding Series 1 Nonconvertible Preferred Stock (Detail) - Series 1 Nonconvertible Preferred Stock [Member] $ in Thousands	Mar. 31, 2019USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,628
Ending Balance	$ 1,628

Marketable Securities - Fair Va

Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 280,702	$ 261,791
Gross Unrealized Gains	78	1
Gross Unrealized Losses	(143)	(575)
Fair Value	280,637	261,217
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	106,648	42,801
Gross Unrealized Gains	29
Gross Unrealized Losses	(7)	(98)
Fair Value	106,670	42,703
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	100,829	113,885
Fair Value	100,829	113,885
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	73,225	105,105
Gross Unrealized Gains	49	1
Gross Unrealized Losses	(136)	(477)
Fair Value	$ 73,138	$ 104,629

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2019	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Marketable securities maturing in greater than one year, aggregate fair value	$ 16,510	$ 16,389
Maximum [Member] | Short Term Marketable Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	1 year
Long Term Marketable Securities [Member] | Minimum [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	1 year
Long Term Marketable Securities [Member] | Maximum [Member] | Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Maturity period of the marketable securities	3 years

Accrued Expenses and Other Lo_3

Accrued Expenses and Other Long-Term Liabilities - Accrued Expenses and Other Current Liabilities as well as Other Long-Term Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Accrued expenses:
Accrued preclinical and clinical expenses	$ 5,530	$ 3,617
Accrued payroll and related expenses	1,720	3,274
Accrued vendor manufacturing	2,695	1,901
Accrued professional fees	742	507
Accrued other	920	593
Accrued expenses	11,607	9,892
Other long-term liabilities:
Uncertain tax positions	1,922	1,792
Accrued rent expense	842	593
Capital lease obligation	249	293
Asset retirement obligation	236	217
Other long-term liabilities	$ 3,249	$ 2,895

Ongoing Collaboration Agreeme_2

Ongoing Collaboration Agreements - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended		149 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Oct. 01, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone revenue recognized	$ 39,631,000	$ 44,049,000	$ 109,517,000	$ 82,158,000
AbbVie [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Cash consideration received under collaboration from sale of preferred stock, research funding payments, milestone payments and royalties					$ 787,000,000
Remaining milestone payments to be earned						$ 0
Collaboration agreement tiered royalty description			From ten percent up to twenty percent, or on a blended basis from the ten percent up to the high teens, on the portion of AbbVie?s calendar year net sales
AbbVie [Member] | JAPAN [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone revenue recognized				$ 15,000,000
Type of cost, good or service [Extensible List]				enta:MilestonePayments

Series 1 Nonconvertible Prefe_2

Series 1 Nonconvertible Preferred Stock - Additional Information (Detail) - Series 1 Nonconvertible Preferred Stock [Member] shares in Thousands	Mar. 31, 2019shares
Class Of Stock [Line Items]
Preferred stock, shares issued	1,931
Preferred stock, shares outstanding	1,931

Stock-Based Awards - Summary of

Stock-Based Awards - Summary of Stock Option Activity Including Performance Based Options (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2019USD ($)$ / sharesshares	Sep. 30, 2018USD ($)$ / sharesshares
Options
Outstanding as of beginning of period | shares	2,624
Granted | shares	537
Exercised | shares	(197)
Forfeited | shares	(32)
Outstanding as of end of period | shares	2,932	2,624
Options vested and expected to vest as of end of period | shares	2,932
Options exercisable as of end of period | shares	1,722
Weighted Average Exercise Price
Outstanding | $ / shares	$ 36.65
Granted | $ / shares	82.21
Exercised | $ / shares	28.59
Forfeited | $ / shares	70.59
Outstanding | $ / shares	45.17	$ 36.65
Options vested and expected to vest as of end of period | $ / shares	45.17
Options exercisable as of end of period | $ / shares	$ 34.47
Weighted Average Remaining Contractual Term
Outstanding as of end of period	7 years 2 months 12 days	7 years 1 month 6 days
Options vested and expected to vest as of end period	7 years 2 months 12 days
Options exercisable as of end of period	6 years 1 month 6 days
Aggregate Intrinsic Value
Outstanding as of end of period | $	$ 148,329	$ 129,115
Options vested and expected to vest as of end of period | $	148,329
Options exercisable as of end of period | $	$ 105,171

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended			6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2016	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate of unrecognized stock-based compensation cost	$ 44,379			$ 44,379
Weighted average recognition period				2 years 7 months 6 days
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate target share issued on vesting, net of share withholding	77	45		77
Relative Total Stockholder Return Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				200.00%
PSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage				80.00%
Restricted Stock Units [Member] | Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			50.00%
Vesting period			3 years
Restricted Stock Units [Member] | Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			50.00%
Vesting period			4 years
Executive Officers [Member] | Relative Total Stockholder Return Units [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares range percentage				0.00%
Executive Officers [Member] | Relative Total Stockholder Return Units [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares range percentage				200.00%

Stock-Based Awards - Summary _2

Stock-Based Awards - Summary of PSUs and rTSRUs Activity Based on Targeted Number of Shares (Detail) shares in Thousands	6 Months Ended
	Mar. 31, 2019$ / sharesshares
PSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance | shares	70
Granted | shares	21
Vested | shares	(36)
Cancelled | shares	(9)
Unvested, ending balance | shares	46
Weighted Average Grant Date Fair Value, Unvested beginning balance | $ / shares	$ 50.97
Weighted Average Grant Date Fair Value, Granted | $ / shares	67.13
Weighted Average Grant Date Fair Value, Vested | $ / shares	35.89
Weighted Average Grant Date Fair Value, Cancelled | $ / shares	35.89
Weighted Average Grant Date Fair Value, Unvested ending balance | $ / shares	$ 73.03
rTSRUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance | shares	70
Granted | shares	21
Vested | shares	(45)
Unvested, ending balance | shares	46
Weighted Average Grant Date Fair Value, Unvested beginning balance | $ / shares	$ 59.96
Weighted Average Grant Date Fair Value, Granted | $ / shares	47.42
Weighted Average Grant Date Fair Value, Vested | $ / shares	46.11
Weighted Average Grant Date Fair Value, Unvested ending balance | $ / shares	$ 67.80

Stock-Based Awards - Summary _3

Stock-Based Awards - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member] shares in Thousands	6 Months Ended
	Mar. 31, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance | shares	109
Granted | shares	0
Vested | shares	0
Cancelled | shares	(1)
Unvested, ending balance | shares	108
Weighted Average Grant Date Fair Value, Unvested beginning balance | $ / shares	$ 30
Weighted Average Grant Date Fair Value, Granted | $ / shares	0
Weighted Average Grant Date Fair Value, Vested | $ / shares	0
Weighted Average Grant Date Fair Value, Cancelled | $ / shares	30
Weighted Average Grant Date Fair Value, Unvested ending balance | $ / shares	$ 30

Stock-Based Awards - Stock-Base

Stock-Based Awards - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 4,707	$ 3,590	$ 10,550	$ 7,710
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	3,949	3,030	7,853	5,978
Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	72	35	1,278	641
rTSRUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	493	332	1,025	694
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	193	193	394	397
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	2,194	1,429	4,468	2,900
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 2,513	$ 2,161	$ 6,082	$ 4,810

Net Income Per Share - Basic an

Net Income Per Share - Basic and Diluted Net Income Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
Basic net income per share:
Net income	$ 4,145	$ 26,011	$ 12,555	$ 11,693	$ 30,156	$ 24,248
Weighted average common shares outstanding—basic	19,549		19,206		19,487	19,167
Net income per share common share—basic	$ 0.21		$ 0.65		$ 1.55	$ 1.27
Diluted net income per share:
Net income	$ 4,145	$ 26,011	$ 12,555	$ 11,693	$ 30,156	$ 24,248
Weighted average common shares outstanding—basic	19,549		19,206		19,487	19,167
Dilutive effect of common stock equivalents	1,535		1,395		1,459	1,089
Weighted average common shares outstanding—diluted	21,084		20,601		20,946	20,256
Net income per share common share—diluted	$ 0.20		$ 0.61		$ 1.44	$ 1.20
Anti-dilutive common stock equivalents excluded from above	784		46		694	449

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income tax (expense) benefit	$ 3,204	$ (5,370)	$ (526)	$ (9,014)
Effective tax rates			1.70%	27.10%
Federal statutory rate			21.00%
Income tax expense as a result of U.S. Tax Cuts and Jobs Act				$ (3,859)
Unrecognized tax benefits	$ 1,922		$ 1,922		$ 1,792

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	6 Months Ended
	Mar. 31, 2019USD ($)Lease	Mar. 31, 2018USD ($)	Sep. 30, 2018USD ($)
Commitments And Contingencies [Line Items]
Number of leases | Lease	2
Rent expense	$ 1,246	$ 1,013
Outstanding letter of credit	608		$ 608
Long term restricted cash	$ 608		$ 608
First Lease [Member] | Watertown, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Office lease expiration period	Sep. 30, 2022
Second Lease [Member] | Watertown, Massachusetts [Member]
Commitments And Contingencies [Line Items]
Office lease expiration period	Aug. 31, 2024