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Fair Value Measurements and Investments - Schedule of Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Millions | Jun. 30, 2025 | Sep. 30, 2024
--- | --- | ---
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | $ 2,964 | $ 5,412
Gross Unrealized Gains | 16 | 44
Gross Unrealized Losses | 0 | (5)
Fair Value | 2,980 | 5,451
U.S. government-sponsored debt securities |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 303 | 492
Gross Unrealized Gains | 1 | 4
Gross Unrealized Losses | 0 | 0
Fair Value | 304 | 496
U.S. Treasury securities |  |
Debt Securities, Available-for-Sale [Line Items] |  |
Amortized Cost | 2,661 | 4,920
Gross Unrealized Gains | 15 | 40
Gross Unrealized Losses | 0 | (5)
Fair Value | $ 2,676 | $ 4,955

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