Organization and Significant Accounting Policies - Narrative (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 02, 2024	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Organization And Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash		$ 127,700		$ 127,700
Restricted cash		2,200		2,200
Available-for-sale securities, at fair value		395,410		395,410		$ 292,735
Increase (decrease) in cash and investments				119,500
Development regulatory and sales milestones payments		2,800,000		2,800,000
Income tax (benefit) expense		0	$ 0	(3,313)	$ 17
Underwriting Agreement
Organization And Significant Accounting Policies [Line Items]
Net proceeds	$ 429,300
Shares of common stock issued (in shares)	15,790,000
Common stock offering price (in dollars per share)	$ 28.50
Aggregate purchase price paid by investors	$ 450,000
Amgen | Royalty Pharma Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment receivable upon achievement of enrollment in phase 3 clinical trial		$ 50,000		$ 50,000