Income Taxes - Reconciliation of Beginning and Ending Unrecognized Tax Benefits by Fiscal Year (Detail) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Sep. 30, 2021 |
Sep. 30, 2020 |
Sep. 30, 2019 |
|
| Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] | |||
| Balance at beginning of period | $ 2,579 | $ 2,234 | $ 1,658 |
| Increases of unrecognized tax benefits related to prior years | 34 | 66 | 216 |
| Decreases of unrecognized tax benefits related to prior years | (386) | (83) | (13) |
| Increases of unrecognized tax benefits related to current year | 326 | 376 | 384 |
| Decreases related to settlements with taxing authorities | (63) | (12) | (9) |
| Reductions related to lapsing statute of limitations | (2) | (2) | (2) |
| Balance at end of period | $ 2,488 | $ 2,579 | $ 2,234 |