Fair Value Measurements and Investments - Amortized Cost, Unrealized Gains and Losses, and Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Millions |
Mar. 31, 2023 |
Sep. 30, 2022 |
|---|---|---|
| Debt Securities, Available-for-Sale [Line Items] | ||
| Amortized Cost | $ 5,348 | $ 5,395 |
| Gross Unrealized Gains | 7 | 0 |
| Gross Unrealized Losses | (90) | (134) |
| Fair Value | 5,265 | 5,261 |
| U.S. government-sponsored debt securities | ||
| Debt Securities, Available-for-Sale [Line Items] | ||
| Amortized Cost | 1,016 | 458 |
| Gross Unrealized Gains | 1 | 0 |
| Gross Unrealized Losses | (1) | (1) |
| Fair Value | 1,016 | 457 |
| U.S. Treasury securities | ||
| Debt Securities, Available-for-Sale [Line Items] | ||
| Amortized Cost | 4,332 | 4,937 |
| Gross Unrealized Gains | 6 | 0 |
| Gross Unrealized Losses | (89) | (133) |
| Fair Value | $ 4,249 | $ 4,804 |