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Organization and Significant Accounting Policies - Narrative (Details) |  |  |  |  |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 13, 2025 USD ($) | Feb. 24, 2025 USD ($) | Feb. 14, 2025 USD ($) | Feb. 07, 2025 USD ($) | Nov. 25, 2024 USD ($) | Aug. 31, 2025 USD ($) | Sep. 30, 2025 USD ($) segment subsidiary | Sep. 30, 2024 USD ($) | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($)
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Cash, cash equivalents and restricted cash |  |  |  |  |  |  | $ 226,548,000 | $ 102,685,000 | $ 110,891,000 | $ 108,005,000
Restricted cash |  |  |  |  |  |  | 1,900,000 | 3,500,000 |  |
Available-for-sale securities, at fair value |  |  |  |  |  |  | 692,818,000 | 578,276,000 |  |
Net increase (decrease) in cash and investments |  |  |  |  |  |  | 238,400,000 |  |  |
Value of stock repurchased |  |  |  |  |  |  | 53,193,000 |  |  |
Interest income |  |  |  |  |  |  | 37,289,000 | 22,720,000 | 15,299,000 |
Proceeds from the issuance of warrants |  |  |  |  | $ 25,000,000.0 |  | 25,000,000 | 0 | 0 |
Repayments of credit facility |  |  |  |  |  |  | 201,625,000 | 0 | 0 |
Capital expenditure |  |  |  |  |  |  | 22,666,000 | 141,469,000 | 176,737,000 |
Development regulatory and sales milestones payments |  |  |  |  |  |  | 13,400,000,000 |  |  |
Intangible assets subject to amortization, impairment |  |  |  |  |  |  | $ 0 | $ 0 | $ 0 |
Number of subsidiaries with foreign currency as functional currency | subsidiary |  |  |  |  |  |  | 3 |  |  |
Number of operating segments | segment |  |  |  |  |  |  | 1 |  |  |
Sarepta | Sarepta Collaboration Agreement |  |  |  |  |  |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |
Proceeds from collaboration arrangements | $ 50,000,000 | $ 500,000,000 | $ 500,000,000 |  |  | $ 50,000,000.0 |  |  |  |
Proceeds from issuance of equity |  |  |  | $ 325,000,000.0 |  |  |  |  |  |
Development milestone payments |  |  |  |  |  | 100,000,000.0 |  |  |  |
Value of stock repurchased | $ 50,000,000 |  |  |  |  | $ 50,000,000 |  |  |  |

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481830/320-10-45-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479130/326-30-45-1 + Details
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