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Debt - Summary of Outstanding Term Loans (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
May 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]        
Credit facility maximum borrowing capacity $ 3,490   $ 3,306 $ 2,662
Borrowings outstanding 317   35  
2017 Term Loan [Member]        
Debt Instrument [Line Items]        
Credit facility maximum borrowing capacity   $ 1,000    
Borrowings outstanding 1,500   800  
Term loans paid down $ 1,200   $ 1,200