Organization and Significant Accounting Policies - Narrative (Details) - USD ($) $ / shares in Units, shares in Thousands			2 Months Ended	12 Months Ended
	Aug. 07, 2024	Jan. 02, 2024	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Organization And Significant Accounting Policies [Line Items]
Cash, cash equivalents and restricted cash			$ 102,685,000	$ 102,685,000	$ 110,891,000	$ 108,005,000	$ 184,434,000
Restricted cash			3,500,000	3,500,000	7,900,000
Available-for-sale securities, at fair value			578,276,000	578,276,000	292,735,000
Net increase (decrease) in cash and investments				277,300,000
Development regulatory and sales milestones payments			2,700,000,000	2,700,000,000
Intangible assets subject to amortization, impairment				0	$ 0	$ 0
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account			250,000	$ 250,000
Underwriting Agreement
Organization And Significant Accounting Policies [Line Items]
Shares of common stock issued (in shares)		15,790
Common stock offering price (in dollars per share)		$ 28.50
Aggregate purchase price by investors		$ 450,000,000.0
Net proceeds		$ 429,300,000
Financing Agreement | Line of Credit
Organization And Significant Accounting Policies [Line Items]
Debt instrument, term	7 years
Proceeds from debt, net of issuance costs	$ 390,700,000		$ 388,900,000
Secured Debt | Financing Agreement | Line of Credit
Organization And Significant Accounting Policies [Line Items]
Maximum borrowing capacity	500,000,000
Long-term line of credit	400,000,000
Line of credit facility, remaining borrowing capacity	$ 100,000,000