Fair Value Measurements and Investments - Schedule of Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Debt Securities (Details) - USD ($) $ in Millions |
Mar. 31, 2026 |
Sep. 30, 2025 |
|---|---|---|
| Debt Securities, Available-for-Sale [Line Items] | ||
| Amortized Cost | $ 1,386 | $ 2,405 |
| Gross Unrealized Gains | 7 | 16 |
| Gross Unrealized Losses | 0 | 0 |
| Fair Value | 1,393 | 2,421 |
| U.S. government-sponsored debt securities | ||
| Debt Securities, Available-for-Sale [Line Items] | ||
| Amortized Cost | 129 | 304 |
| Gross Unrealized Gains | 0 | 1 |
| Gross Unrealized Losses | 0 | 0 |
| Fair Value | 129 | 305 |
| U.S. Treasury securities | ||
| Debt Securities, Available-for-Sale [Line Items] | ||
| Amortized Cost | 1,257 | 2,101 |
| Gross Unrealized Gains | 7 | 15 |
| Gross Unrealized Losses | 0 | 0 |
| Fair Value | $ 1,264 | $ 2,116 |
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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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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). Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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