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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Dec. 31, 2018	Jan. 31, 2019
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
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		19,441,584

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Current assets:
Cash and cash equivalents	$ 74,365	$ 63,902
Short-term marketable securities	271,423	244,828
Accounts receivable	69,886	67,205
Prepaid expenses and other current assets	7,636	4,454
Total current assets	423,310	380,389
Long-term marketable securities	11,465	16,389
Property and equipment, net	9,493	8,374
Deferred tax assets	9,248	8,375
Restricted cash	608	608
Other long-term assets	92	92
Total assets	454,216	414,227
Current liabilities:
Accounts payable	5,622	4,745
Accrued expenses and other current liabilities	11,152	9,892
Income taxes payable	5,858	1,388
Total current liabilities	22,632	16,025
Series 1 nonconvertible preferred stock	1,628	1,628
Other long-term liabilities	3,121	2,895
Total liabilities	27,381	20,548
Commitments and contingencies (Note 11)
Stockholders' equity:
Common stock; $0.01 par value per share, 100,000 shares authorized; 19,435 and 19,395 shares issued and outstanding at December 31, 2018 and September 30, 2018, respectively	194	194
Additional paid-in capital	283,530	276,526
Accumulated other comprehensive loss	(257)	(398)
Retained earnings	143,368	117,357
Total stockholders' equity	426,835	393,679
Total liabilities and stockholders' equity	$ 454,216	$ 414,227

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	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	19,435,000	19,395,000
Common stock, shares outstanding	19,435,000	19,395,000

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue
Total revenue	$ 69,886	$ 38,109
Operating expenses:
Research and development	34,878	17,962
General and administrative	7,152	5,770
Total operating expenses	42,030	23,732
Income from operations	27,856	14,377
Other income (expense):
Interest income (expense), net	1,885	919
Change in fair value of warrant liability and Series 1 nonconvertible preferred stock		41
Total other income (expense), net	1,885	960
Income before income taxes	29,741	15,337
Income tax expense	(3,730)	(3,644)
Net income	$ 26,011	$ 11,693
Net income per share:
Basic	$ 1.34	$ 0.61
Diluted	$ 1.25	$ 0.59
Weighted average shares outstanding:
Basic	19,426	19,130
Diluted	20,810	19,918
Royalties [Member]
Revenue
Total revenue	$ 69,886	$ 23,109
Milestone [Member]
Revenue
Total revenue		$ 15,000

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 26,011	$ 11,693
Other comprehensive income (loss):
Net unrealized gains (losses) on marketable securities, net of tax of ($45) and ($107)	141	(346)
Total other comprehensive income (loss)	141	(346)
Comprehensive income	$ 26,152	$ 11,347

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net unrealized gains (losses) on marketable securities, tax	$ (45)	$ (107)

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 26,011	$ 11,693
Adjustments to reconcile net income to net cash provided by operating activities:
Stock-based compensation expense	5,843	4,120
Depreciation and amortization expense	680	587
Deferred income taxes	(918)	2,662
Premium paid on marketable securities		(1)
Amortization of (accretion of) premium (discount) on marketable securities	(977)	88
Change in fair value of warrant liability and Series 1 nonconvertible preferred stock		(41)
Other non-cash items	39
Change in operating assets and liabilities:
Accounts receivable	(2,681)	(12,495)
Prepaid expenses and other current assets	(3,182)	(539)
Accounts payable	124	(129)
Accrued expenses	598	(2,494)
Income taxes payable	4,470	959
Other long-term liabilities	247	(1)
Net cash provided by operating activities	30,254	4,409
Cash flows from investing activities
Purchase of marketable securities	(119,546)	(54,710)
Proceeds from maturities and sale of marketable securities	98,999	52,766
Purchase of property and equipment	(384)	(504)
Net cash used in investing activities	(20,931)	(2,448)
Cash flows from financing activities
Proceeds from exercise of stock options and warrants	1,161	436
Payments of capital lease obligations	(21)	(19)
Net cash provided by financing activities	1,140	417
Net increase in cash, cash equivalents and restricted cash	10,463	2,378
Cash, cash equivalents and restricted cash at beginning of period	64,510	66,283
Cash, cash equivalents and restricted cash at end of period	74,973	68,661
Supplemental disclosure of cash flow information:
Cash paid for income taxes	5	13
Supplemental disclosure of non-cash investing information:
Purchases of fixed assets included in accounts payable and accrued expenses	$ 1,304	$ 111

Nature of the Business and Basis of Presentation

3 Months Ended

Dec. 31, 2018

Accounting Policies [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). Royalties from the Company’s AbbVie collaboration and its existing financial resources provide funding to support its wholly-owned research and development programs, which are currently focused on the following disease targets: respiratory syncytial virus (“RSV”), non-alcoholic steatohepatitis (“NASH”); primary biliary cholangitis (“PBC”); and hepatitis B virus (“HBV”).

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 December 31, 2018 and for the three months ended December 31, 2018 and 2017 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 December 31, 2018 and results of operations for the three months ended December 31, 2018 and 2017 and cash flows for the three months ended December 31, 2018 and 2017, have been made. The results of operations for the three months ended December 31, 2018 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 Accounting Policies

3 Months Ended

Dec. 31, 2018

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

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. The Company will apply the guidance in ASC 606 to its one ongoing collaboration agreement under which it is eligible to earn sales-based royalties and to qualifying arrangements entered into after October 1, 2018. The Company recognizes revenue related to the sales-based royalties in the period in which the underlying sales occur based on actual sales as communicated by the licensee. The license is deemed to be the predominant item to which the royalties relate.

Recently Issued Accounting Pronouncements

In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”) and has since issued several additional amendments thereto, collectively referred to herein as ASC 606, which supersedes the revenue recognition requirements in ASC 605-25, Multiple-Element Arrangements and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The update also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. Companies have the option of applying this new guidance retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application. This guidance was effective for the Company in the fiscal year beginning October 1, 2018. The Company adopted ASC 606 as of October 1, 2018 using the modified retrospective transition method. The adoption did not have an impact on its financial statements as the Company had satisfied its performance obligations under its one open revenue contract in fiscal 2011, prior to the adoption of ASC 606. The adoption of this guidance did not have an impact on the Company’s accounting for royalty payments as the Company receives sales-based royalties for which the license is deemed to be the predominant item to which the royalties relate.

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. Upon the adoption of ASU 2016-18, the amount of cash and cash equivalents previously presented in the consolidated statements of cash flows for the three months ended December 31, 2017 increased by $608 as of beginning and end of the period to reflect the inclusion of restricted cash in the amount reported for changes in cash, cash equivalents and restricted cash.

In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) (“ASU 2017-09”) which provides updated guidance about changes to the terms or conditions of a share-based payment award that requires companies to apply modification accounting under Topic 718. This amendment was effective for the Company in the fiscal year beginning October 1, 2018. The Company adopted ASU 2017-09 as of October 1, 2018. The adoption of ASU 2017-09 did not have an impact on its consolidated financial statements.

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 and Liabilities

3 Months Ended

Dec. 31, 2018

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 December 31, 2018 and September 30, 2018 and indicate the fair value hierarchy of the valuation inputs utilized to determine such fair value:

	Fair Value Measurements at December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash equivalents:
Money market funds	$	42,589	$	—	$	—	$	42,589
U.S. Treasury notes		9,981		—		—		9,981
Commercial paper		—		17,236		—		17,236
Corporate bonds		—		2,498		—		2,498
Marketable securities:
U.S. Treasury notes		50,742		—		—		50,742
Commercial paper		—		135,713		—		135,713
Corporate bonds		—		96,433		—		96,433
	$	103,312	$	251,880	$	—	$	355,192
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 three months ended December 31, 2018 and 2017, 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)
	December 31,	September 30,
Unobservable Input	2018	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, December 31, 2018	$	1,628

- Definition

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

+ References + Details

Marketable Securities

3 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Marketable Securities

4.	Marketable Securities

As of December 31, 2018 and September 30, 2018, the fair value of available-for-sale marketable securities, by type of security, was as follows:

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Commercial paper	$	135,713	$	—	$	—			135,713
Corporate bonds		96,776		7		(350	)		96,433
U.S. Treasury notes		50,787		—		(45	)		50,742
	$	283,276	$	7	$	(395	)	$	282,888

	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 December 31, 2018, 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 $11,465, 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-Term Liabilities

3 Months Ended

Dec. 31, 2018

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 December 31, 2018 and September 30, 2018:

	December 31,		September 30,
	2018		2018
	(in thousands)
Accrued expenses:
Accrued preclinical and clinical expenses	$	5,396	$	3,617
Accrued payroll and related expenses		1,448		3,274
Accrued vendor manufacturing		2,577		1,901
Accrued professional fees		696		507
Accrued other		1,035		593
	$	11,152	$	9,892

Other long-term liabilities:
Uncertain tax positions	$	1,950	$	1,792
Accrued rent expense		674		593
Capital lease obligation		271		293
Asset retirement obligation		226		217
	$	3,121	$	2,895

Ongoing Collaboration Agreements

3 Months Ended

Dec. 31, 2018

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 $717,000 through December 31, 2018. 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 three months ended December 31, 2017, 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. As of October 1, 2018, the adoption date of ASC 606, there were no remaining milestone payments to be earned under this agreement.

The Company is also receiving annually tiered royalties per Company protease 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 Preferred Stock

3 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Series 1 Nonconvertible Preferred Stock

7.	Series 1 Nonconvertible Preferred Stock

As of December 31, 2018, 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

3 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Awards

8.	Stock-Based Awards

The Company has granted stock-based awards, including stock options, restricted stock units, and performance share units, under its existing 2012 Equity Incentive Plan (the “2012 Plan”). The Company also has outstanding stock-based awards under its amended and restated 1995 Equity Incentive Plan (the “1995 Plan”), but is no longer granting awards under this plan.

The following table summarizes stock option activity, including performance-based options, for the year-to-date period ending December 31, 2018:

	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		484			81.04
Exercised		(40	)		28.76
Forfeited		(6	)		90.95
Outstanding as of December 31, 2018		3,062		$	43.67		7.3	$	90,501
Options vested and expected to vest as of December 31, 2018		3,062		$	43.67		7.3	$	90,501
Options exercisable as of December 31, 2018		1,739		$	32.66		6.1	$	66,654

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. The following table summarizes PSU and rTSRU activity for the year-to-date period ending December 31, 2018:

	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		—		—		—		—
Cancelled		—		—		—		—
Unvested at December 31, 2018		91	$	54.68		91	$	57.08

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 December 31, 2018:

	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		—		—
Unvested at December 31, 2018		109	$	30.00

Stock-Based Compensation Expense

During the three months ended December 31, 2018 and 2017, the Company recognized the following stock-based compensation expense:

	Three Months ended
	December 31,
	2018		2017
	(in thousands)
Research and development	$	2,274	$	1,471
General and administrative		3,569		2,649
	$	5,843	$	4,120

	Three Months ended
	December 31,
	2018		2017
	(in thousands)
Stock options	$	3,904	$	2,948
Performance stock units		1,206		606
rTSRUs		531		362
Restricted stock units		202		204
	$	5,843	$	4,120

During the three months ended December 31, 2018 and 2017, 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 December 31, 2018, the Company had an aggregate of $48,700 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.8 years.

Net Income Per Share

3 Months Ended

Dec. 31, 2018

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 three months ended December 31, 2018 and 2017 (in thousands, except per share data):

	Three Months Ended
	December 31,
	2018		2017
	(in thousands, except per share data)
Basic net income per share:
Numerator:
Net income	$	26,011	$	11,693
Denominator:
Weighted average common shares outstanding—basic		19,426		19,130
Net income per share common share—basic	$	1.34	$	0.61
Diluted net income per share:
Numerator:
Net income	$	26,011	$	11,693
Denominator:
Weighted average common shares outstanding—basic		19,426		19,130
Dilutive effect of common stock equivalents		1,384		788
Weighted average common shares outstanding—diluted		20,810		19,918
Net income per share common share—diluted	$	1.25	$	0.59
Anti-dilutive common stock equivalents excluded from above		606		842

Income Taxes

3 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

10.	Income Taxes

For the three months ended December 31, 2018 and 2017, the Company recorded an income tax expense of $3,730 and $3,644, respectively, which was attributable to the Company’s domestic operations. During the three months ended December 31, 2017, income tax expense included a revaluation adjustment against deferred tax assets of $(3,859) due to a decrease in the federal corporate income tax rate as enacted under the U.S. Tax Cuts and Jobs Act (the “Tax Act”).

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 $1,950 and $1,792 as of December 31, 2018 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

3 Months Ended

Dec. 31, 2018

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. Free rent and payment escalations specified in the lease agreements are accrued such that rent expense is recognized on a straight-line basis over the term of occupancy. The Company recorded rent expense of $623 and $506 for the three months ended December 31, 2018 and 2017, respectively.

In connection with one of the leases, the Company has a total outstanding letter of credit in the amount of $608 as of December 31, 2018 and September 30, 2018, collateralized by a money market account. As of December 31, 2018 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 December 31, 2018.

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2018
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. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Actual results could differ from the Company’s estimates.
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. The Company will apply the guidance in ASC 606 to its one ongoing collaboration agreement under which it is eligible to earn sales-based royalties and to qualifying arrangements entered into after October 1, 2018. The Company recognizes revenue related to the sales-based royalties in the period in which the underlying sales occur based on actual sales as communicated by the licensee. The license is deemed to be the predominant item to which the royalties relate.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In May 2014, the FASB issued ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606) (“ASU 2014-09”) and has since issued several additional amendments thereto, collectively referred to herein as ASC 606, which supersedes the revenue recognition requirements in ASC 605-25, Multiple-Element Arrangements and most industry-specific guidance. The new standard requires that an entity recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The update also requires additional disclosure about the nature, amount, timing, and uncertainty of revenue and cash flows arising from customer contracts, including significant judgments and changes in judgments and assets recognized from costs incurred to obtain or fulfill a contract. Companies have the option of applying this new guidance retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initially applying this update recognized at the date of initial application. This guidance was effective for the Company in the fiscal year beginning October 1, 2018. The Company adopted ASC 606 as of October 1, 2018 using the modified retrospective transition method. The adoption did not have an impact on its financial statements as the Company had satisfied its performance obligations under its one open revenue contract in fiscal 2011, prior to the adoption of ASC 606. The adoption of this guidance did not have an impact on the Company’s accounting for royalty payments as the Company receives sales-based royalties for which the license is deemed to be the predominant item to which the royalties relate. 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. Upon the adoption of ASU 2016-18, the amount of cash and cash equivalents previously presented in the consolidated statements of cash flows for the three months ended December 31, 2017 increased by $608 as of beginning and end of the period to reflect the inclusion of restricted cash in the amount reported for changes in cash, cash equivalents and restricted cash. In May 2017, the FASB issued ASU No. 2017-09, Compensation—Stock Compensation (Topic 718) (“ASU 2017-09”) which provides updated guidance about changes to the terms or conditions of a share-based payment award that requires companies to apply modification accounting under Topic 718. This amendment was effective for the Company in the fiscal year beginning October 1, 2018. The Company adopted ASU 2017-09 as of October 1, 2018. The adoption of ASU 2017-09 did not have an impact on its consolidated financial statements. 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 and Liabilities (Tables)

3 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis

	Fair Value Measurements at December 31, 2018 Using:
	Level 1		Level 2		Level 3		Total
	(in thousands)
Assets:
Cash equivalents:
Money market funds	$	42,589	$	—	$	—	$	42,589
U.S. Treasury notes		9,981		—		—		9,981
Commercial paper		—		17,236		—		17,236
Corporate bonds		—		2,498		—		2,498
Marketable securities:
U.S. Treasury notes		50,742		—		—		50,742
Commercial paper		—		135,713		—		135,713
Corporate bonds		—		96,433		—		96,433
	$	103,312	$	251,880	$	—	$	355,192
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

	Range (Weighted Average)
	December 31,	September 30,
Unobservable Input	2018	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, December 31, 2018	$	1,628

Marketable Securities (Tables)

3 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Fair Value of Available-for-Sale Marketable Securities by Type of Security

As of December 31, 2018 and September 30, 2018, the fair value of available-for-sale marketable securities, by type of security, was as follows:

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
Commercial paper	$	135,713	$	—	$	—			135,713
Corporate bonds		96,776		7		(350	)		96,433
U.S. Treasury notes		50,787		—		(45	)		50,742
	$	283,276	$	7	$	(395	)	$	282,888

	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 Long-Term Liabilities (Tables)

3 Months Ended

Dec. 31, 2018

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 December 31, 2018 and September 30, 2018:

	December 31,		September 30,
	2018		2018
	(in thousands)
Accrued expenses:
Accrued preclinical and clinical expenses	$	5,396	$	3,617
Accrued payroll and related expenses		1,448		3,274
Accrued vendor manufacturing		2,577		1,901
Accrued professional fees		696		507
Accrued other		1,035		593
	$	11,152	$	9,892

Other long-term liabilities:
Uncertain tax positions	$	1,950	$	1,792
Accrued rent expense		674		593
Capital lease obligation		271		293
Asset retirement obligation		226		217
	$	3,121	$	2,895

Stock-Based Awards (Tables)

3 Months Ended

Dec. 31, 2018

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 December 31, 2018:

	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		484			81.04
Exercised		(40	)		28.76
Forfeited		(6	)		90.95
Outstanding as of December 31, 2018		3,062		$	43.67		7.3	$	90,501
Options vested and expected to vest as of December 31, 2018		3,062		$	43.67		7.3	$	90,501
Options exercisable as of December 31, 2018		1,739		$	32.66		6.1	$	66,654

Summary of Restricted Stock Unit Activity

The following table summarizes the restricted stock unit activity for the year-to-date period ending December 31, 2018:

	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		—		—
Unvested at December 31, 2018		109	$	30.00

Stock-Based Compensation Expense

During the three months ended December 31, 2018 and 2017, the Company recognized the following stock-based compensation expense:

	Three Months ended
	December 31,
	2018		2017
	(in thousands)
Research and development	$	2,274	$	1,471
General and administrative		3,569		2,649
	$	5,843	$	4,120

	Three Months ended
	December 31,
	2018		2017
	(in thousands)
Stock options	$	3,904	$	2,948
Performance stock units		1,206		606
rTSRUs		531		362
Restricted stock units		202		204
	$	5,843	$	4,120

Performance Share Units and Relative Total Stockholder Return Units [Member]

Summary of PSUs and rTSRUs Activity

The following table summarizes PSU and rTSRU activity for the year-to-date period ending December 31, 2018:

	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		—		—		—		—
Cancelled		—		—		—		—
Unvested at December 31, 2018		91	$	54.68		91	$	57.08

Net Income Per Share (Tables)

3 Months Ended

Dec. 31, 2018

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 three months ended December 31, 2018 and 2017 (in thousands, except per share data):

	Three Months Ended
	December 31,
	2018		2017
	(in thousands, except per share data)
Basic net income per share:
Numerator:
Net income	$	26,011	$	11,693
Denominator:
Weighted average common shares outstanding—basic		19,426		19,130
Net income per share common share—basic	$	1.34	$	0.61
Diluted net income per share:
Numerator:
Net income	$	26,011	$	11,693
Denominator:
Weighted average common shares outstanding—basic		19,426		19,130
Dilutive effect of common stock equivalents		1,384		788
Weighted average common shares outstanding—diluted		20,810		19,918
Net income per share common share—diluted	$	1.25	$	0.59
Anti-dilutive common stock equivalents excluded from above		606		842

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	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 Assets and Liabilities - Financial Assets and Liabilities that were Subject to Fair Value Measurement on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Assets:
Marketable securities	$ 282,888	$ 261,217
Assets, Fair Value Disclosure, Total	355,192	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	103,312	93,728
Level 2 [Member]
Assets:
Assets, Fair Value Disclosure, Total	251,880	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
U.S. Treasury Notes [Member]
Assets:
Cash equivalents	9,981
Marketable securities	50,742	42,703
U.S. Treasury Notes [Member] \| Level 1 [Member]
Assets:
Cash equivalents	9,981
Marketable securities	50,742	42,703
Money Market Funds [Member]
Assets:
Cash equivalents	42,589	51,025
Money Market Funds [Member] \| Level 1 [Member]
Assets:
Cash equivalents	42,589	51,025
Corporate Bonds [Member]
Assets:
Cash equivalents	2,498	3,998
Marketable securities	96,433	104,629
Corporate Bonds [Member] \| Level 2 [Member]
Assets:
Cash equivalents	2,498	3,998
Marketable securities	96,433	104,629
Commercial Paper [Member]
Assets:
Cash equivalents	17,236	6,987
Marketable securities	135,713	113,885
Commercial Paper [Member] \| Level 2 [Member]
Assets:
Cash equivalents	17,236	6,987
Marketable securities	$ 135,713	$ 113,885

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Transfers between Level 1, Level 2 and Level 3	$ 0	$ 0

Fair Value of Financial Assets and Liabilities - Fair Value Measurements of the Company's Outstanding Series 1 Nonconvertible Preferred Stock (Detail) - Level 3 [Member]	Dec. 31, 2018	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)	0.00	0.00
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 Assets and Liabilities - Rollforward of Aggregate Fair Values of Outstanding Series 1 Nonconvertible Preferred Stock (Detail) - Series 1 Nonconvertible Preferred Stock [Member] $ in Thousands	Dec. 31, 2018 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,628
Ending Balance	$ 1,628

Marketable Securities - Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 283,276	$ 261,791
Gross Unrealized Gains	7	1
Gross Unrealized Losses	(395)	(575)
Fair Value	282,888	261,217
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	135,713	113,885
Fair Value	135,713	113,885
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	96,776	105,105
Gross Unrealized Gains	7	1
Gross Unrealized Losses	(350)	(477)
Fair Value	96,433	104,629
U.S. Treasury Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,787	42,801
Gross Unrealized Losses	(45)	(98)
Fair Value	$ 50,742	$ 42,703

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Sep. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Marketable securities maturing in greater than one year, aggregate fair value	$ 11,465	$ 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 Long-Term Liabilities - Accrued Expenses and Other Current Liabilities as well as Other Long-Term Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Accrued expenses:
Accrued preclinical and clinical expenses	$ 5,396	$ 3,617
Accrued payroll and related expenses	1,448	3,274
Accrued vendor manufacturing	2,577	1,901
Accrued professional fees	696	507
Accrued other	1,035	593
Accrued expenses	11,152	9,892
Other long-term liabilities:
Uncertain tax positions	1,950	1,792
Accrued rent expense	674	593
Capital lease obligation	271	293
Asset retirement obligation	226	217
Other long-term liabilities	$ 3,121	$ 2,895

Ongoing Collaboration Agreements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		146 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Oct. 01, 2018
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone revenue recognized	$ 69,886	$ 38,109
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			$ 717,000
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
Type of cost, good or service [Extensible List]		enta:MilestonePayments

Series 1 Nonconvertible Preferred Stock - Additional Information (Detail) - Series 1 Nonconvertible Preferred Stock [Member] shares in Thousands	Dec. 31, 2018 shares
Class Of Stock [Line Items]
Preferred stock, shares issued	1,931
Preferred stock, shares outstanding	1,931

Stock-Based Awards - Summary of Stock Option Activity Including Performance Based Options (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018
Options
Outstanding as of beginning of period	2,624
Granted	484
Exercised	(40)
Forfeited	(6)
Outstanding as of end of period	3,062	2,624
Options vested and expected to vest as of end of period	3,062
Options exercisable as of end of period	1,739
Weighted Average Exercise Price
Outstanding	$ 36.65
Granted	81.04
Exercised	28.76
Forfeited	90.95
Outstanding	43.67	$ 36.65
Options vested and expected to vest as of end of period	43.67
Options exercisable as of end of period	$ 32.66
Weighted Average Remaining Contractual Term
Outstanding as of end of period	7 years 3 months 18 days	7 years 1 month 6 days
Options vested and expected to vest as of end period	7 years 3 months 18 days
Options exercisable as of end of period	6 years 1 month 6 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value	$ 90,501	$ 129,115
Options exercisable as of end of period	66,654
Options vested and expected to vest as of December 31, 2018	$ 90,501

Stock-Based Awards - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate of unrecognized stock-based compensation cost	$ 48,700
Weighted average recognition period	2 years 9 months 18 days
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 of PSUs and rTSRUs Activity (Detail) shares in Thousands	3 Months Ended
	Dec. 31, 2018 $ / shares shares
PSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance \| shares	70
Granted \| shares	21
Unvested, ending balance \| shares	91
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, Unvested ending balance \| $ / shares	$ 54.68
rTSRUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance \| shares	70
Granted \| shares	21
Unvested, ending balance \| shares	91
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, Unvested ending balance \| $ / shares	$ 57.08

Stock-Based Awards - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member] shares in Thousands	3 Months Ended
	Dec. 31, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance \| shares	109
Granted \| shares	0
Vested \| shares	0
Cancelled \| shares	0
Unvested, ending balance \| shares	109
Weighted Average Grant Date Fair Value, Unvested beginning balance \| $ / shares	$ 30.00
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	0
Weighted Average Grant Date Fair Value, Unvested ending balance \| $ / shares	$ 30.00

Stock-Based Awards - Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 5,843	$ 4,120
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	3,904	2,948
Performance Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	1,206	606
rTSRUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	531	362
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	202	204
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	2,274	1,471
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 3,569	$ 2,649

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
	Dec. 31, 2018	Dec. 31, 2017
Basic net income per share:
Net income	$ 26,011	$ 11,693
Weighted average common shares outstanding—basic	19,426	19,130
Net income per share common share—basic	$ 1.34	$ 0.61
Diluted net income per share:
Net income	$ 26,011	$ 11,693
Weighted average common shares outstanding—basic	19,426	19,130
Dilutive effect of common stock equivalents	1,384	788
Weighted average common shares outstanding—diluted	20,810	19,918
Net income per share common share—diluted	$ 1.25	$ 0.59
Anti-dilutive common stock equivalents excluded from above	606	842

- Definition

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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+ Details

- Definition

The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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+ Details

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income tax expense	$ 3,730	$ 3,644
Income tax expense as a result of U.S. Tax Cuts and Jobs Act		$ (3,859)
Unrecognized tax benefits	$ 1,950		$ 1,792

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	3 Months Ended
	Dec. 31, 2018 USD ($) Lease	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)
Commitments And Contingencies [Line Items]
Number of leases \| Lease	2
Rent expense	$ 623	$ 506
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

Stock-Based Awards - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units [Member] shares in Thousands	3 Months Ended
	Dec. 31, 2018 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested, beginning balance \| shares	109
Granted \| shares	0
Vested \| shares	0
Cancelled \| shares	0
Unvested, ending balance \| shares	109
Weighted Average Grant Date Fair Value, Unvested beginning balance \| $ / shares	$ 30.00
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	0
Weighted Average Grant Date Fair Value, Unvested ending balance \| $ / shares	$ 30.00