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Income Taxes - Reconciliation of Unrecognized Tax Benefits Including Interest and Penalties (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning Balance $ 12 $ 18 $ 19
Additions for current year tax positions     3
Additions for prior year tax positions 3 1 3
Subtractions for prior year tax positions (3) (7) (7)
Ending Balance $ 12 $ 12 $ 18