Income Taxes - Additional Information (Detail) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2020 |
Jun. 30, 2019 |
Jun. 30, 2020 |
Jun. 30, 2019 |
|
| Income Tax Disclosure [Abstract] | ||||
| Effective income tax rate reconciliation, percent | 19.00% | 20.00% | 19.00% | 19.00% |
| Increase in unrecognized tax benefits | $ 55 | $ 230 | ||
| Unrecognized tax benefits that would favorably impact effective tax rate | 31 | 70 | ||
| Increase in accrued interest related to uncertain tax positions | $ 18 | $ 19 | $ 56 | $ 51 |