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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

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