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Income Taxes - Reconciliation of Beginning and Ending Unrecognized Tax Benefits by Fiscal Year (Detail) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Balance as of beginning of period $ 2,683 $ 2,488 $ 2,579
Increase in unrecognized tax benefits related to prior years 515 10 34
Decrease in unrecognized tax benefits related to prior years (190) (143) (386)
Increase in unrecognized tax benefits related to current year 510 350 326
Decrease related to settlements with taxing authorities (17) (19) (63)
Reduction related to lapsing statute of limitations (4) (3) (2)
Balance as of end of period $ 3,497 $ 2,683 $ 2,488