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Debt - December 2017 Senior Term Loan Credit Agreement Amendment (Details) - USD ($)
$ in Millions
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Debt Instrument [Line Items]      
Gain (loss) on extinguishment of debt $ (34) $ (7) $ (31)
December Two Thousand Seventeen Senior Term Loan Credit Agreement Amendment      
Debt Instrument [Line Items]      
Gain (loss) on extinguishment of debt $ (1)