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Debt - Schedule of Fair Value of Senior Notes (Detail) - Senior Notes [Member] - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Senior Notes, Fair Value $ 6,518.2 $ 6,911.2
Senior Notes, Book Value $ 6,687.7 $ 6,966.6