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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
6 Months Ended
Mar. 31, 2021
Sep. 30, 2020
Level 2 measurement    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of debt $ 3,371 $ 3,137
Level 3 | Acquisition-related Costs    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Additions to net liabilities $ 7