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Debt - June 2018 Senior Term Loan Credit Agreement Amendment (Details) - USD ($)
$ in Millions
12 Months Ended
Jun. 07, 2018
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Debt Instrument [Line Items]        
Gain (loss) on extinguishment of debt   $ (34) $ (7) $ (31)
June Two Thousand Eighteen Senior Term Loan Credit Agreement Amendment        
Debt Instrument [Line Items]        
Gain (loss) on extinguishment of debt $ (2)