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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, 2021
USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]  
Beginning balance $ (6)
Additions (13)
Reductions 0
Payments 0
Ending balance $ (19)