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Debt - March 2018 Senior Term Loan Credit Agreement Amendment - Additional Information (Detail) - USD ($)
Mar. 31, 2019
Sep. 30, 2018
Mar. 14, 2018
Debt Instrument [Line Items]      
Face or principal amount of debt instrument $ 3,022,000,000 $ 2,851,000,000  
March 2018 Senior Term Loan Credit Agreement Amendment | Acquisition Corp.      
Debt Instrument [Line Items]      
Face or principal amount of debt instrument     $ 320,000,000
Revolving credit facility outstanding     $ 1,326,000,000