Organization and Significant Accounting Policies - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	6 Months Ended
	Feb. 24, 2025	Feb. 07, 2025	Mar. 31, 2025	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2023
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash, cash equivalents and restricted cash			$ 185,709	$ 185,709	$ 102,685	$ 127,704	$ 110,891
Restricted cash			2,100	2,100
Available-for-sale securities			911,700	911,700	$ 578,276
Increase (decrease) in cash and investments				416,400
Development regulatory and sales milestones payments			13,300,000	$ 13,300,000
Sarepta | Sarepta Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from sale of stock		$ 325,000
Shares of common stock issued (in shares)		11,926,301
Common stock sold price (in dollars per share)		$ 27.25
Upfront payment	$ 500,000
Annual fees		$ 250,000
Annual installments period		5 years
Enrollment related milestone payments		$ 300,000
Future potential milestone payments		$ 10,000,000
GSK | GSK-HBV Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development milestone payments			$ 2,500