Fair Value Measurements and Investments - Continuous Unrealized Losses for Less than 12 Months and More than 12 Months (Details) - USD ($) $ in Millions |
Mar. 31, 2023 |
Sep. 30, 2022 |
|---|---|---|
| Debt Securities, Available-for-Sale [Line Items] | ||
| Less Than 12 Months, Fair Value | $ 1,061 | $ 3,915 |
| Less Than 12 Months, Gross Unrealized Losses | (12) | (134) |
| 12 Months or Greater, Fair Value | 2,270 | |
| 12 Months or Greater, Gross Unrealized Losses | (78) | |
| U.S. government-sponsored debt securities | ||
| Debt Securities, Available-for-Sale [Line Items] | ||
| Less Than 12 Months, Fair Value | 179 | 408 |
| Less Than 12 Months, Gross Unrealized Losses | (1) | (1) |
| 12 Months or Greater, Fair Value | 0 | |
| 12 Months or Greater, Gross Unrealized Losses | 0 | |
| U.S. Treasury securities | ||
| Debt Securities, Available-for-Sale [Line Items] | ||
| Less Than 12 Months, Fair Value | 882 | 3,507 |
| Less Than 12 Months, Gross Unrealized Losses | (11) | $ (133) |
| 12 Months or Greater, Fair Value | 2,270 | |
| 12 Months or Greater, Gross Unrealized Losses | $ (78) |