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Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Unrecognized Tax Benefits [Roll Forward]      
Beginning balance $ 414 $ 288 $ 193
Additions for tax positions related to the current year 96 108 110
Additions for tax positions of prior years 17 45 12
Reductions for tax positions of prior years (38) (22) (21)
Settlements with taxing authorities (4) 0 0
Statute closings (2) (5) (6)
Ending balance $ 483 $ 414 $ 288