Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
	Nov. 09, 2022	Mar. 31, 2023	Mar. 31, 2023	Sep. 30, 2022
Organization And Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash		$ 135,000	$ 135,000
Restricted cash		7,300	7,300
Short term investments		346,046	346,046	$ 268,391
Long-term investments		78,834	78,834	$ 105,872
Increase in cash and investments			77,600
Development regulatory and sales milestones payments		3,600,000	3,600,000
Horizon and Amgen
Organization And Significant Accounting Policies [Line Items]
Upfront milestone payment received			40,000
Takeda | License and Co-Funding Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment receivable		40,000	40,000
GSK | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Milestone payment receivable		30,000	30,000
Horizon
Organization And Significant Accounting Policies [Line Items]
Upfront milestone payment received		15,000
Amgen
Organization And Significant Accounting Policies [Line Items]
Upfront milestone payment received		$ 25,000
Royalty Pharma
Organization And Significant Accounting Policies [Line Items]
Upfront milestone payment received	$ 250,000		$ 250,000