Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Jan. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Oct. 03, 2018
Organization And Significant Accounting Policies [Line Items]
Cash and cash equivalents		$ 184,434,000	$ 143,583,000
Restricted Cash		2,400,000	1,800,000
Short term investments		56,627,000	86,890,000
Marketable securities		126,700,000
Long term investments		245,595,000	137,487,000
Net increase (decrease) in cash and investments		160,400,000
Fair value of contingent consideration obligation due to discontinuation of clinical trials		$ 0	$ 0
Calculation of effect of dilutive stock options and restricted stock units		0	0	4,748,958
Right-of-use assets		$ 17,346,000	$ 16,137,000
Right-of-use liabilities		$ 25,545,000
ASU No. 2014-09
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]		true
Change in accounting principle, accounting standards update, adoption date		Oct. 01, 2018
Change in accounting principle, accounting standards update, immaterial effect [true false]		true
ASU No. 2016-02
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]		true
Change in accounting principle, accounting standards update, adoption date		Oct. 01, 2019
Change in accounting principle, accounting standards update, immaterial effect [true false]		true
ASU No. 2018-18
Organization And Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted [true false]		true
Change in accounting principle, accounting standards update, adoption date		Oct. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect [true false]		true
Stock Options
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units				1,007,500
Restricted Stock Units
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units				11,500
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account		$ 250,000
Property and equipment, useful life		7 years
Minimum
Organization And Significant Accounting Policies [Line Items]
Property and equipment, useful life		3 years
Takeda License Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement	$ 300,000,000.0
Horizon License Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement		$ 40,000,000
Janssen
Organization And Significant Accounting Policies [Line Items]
Milestone payment		20,000,000.0			$ 70,000,000.0
Development regulatory and sales milestones payments		5,300,000,000
Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement		$ 340,000,000.0
Janssen | Collaboration Agreement | Maximum
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments					$ 1,900,000,000