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Fair Value Measurements - Reconciliation of Net Liabilities Classified as Level 3 (Details) - Level 3
$ in Millions
12 Months Ended
Sep. 30, 2020
USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Beginning balance $ (9)
Additions (6)
Reductions 7
Payments 2
Ending balance $ (6)