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Fair Values of Certain Financial Liabilities and Other Financial Instruments (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Financial liabilities    
Senior notes $ 2,780 $ 2,778
Credit facility, carrying value 1,442 1,170
Quoted Prices in Active Markets for Identical Assets (Level 1) | Senior Notes    
Financial liabilities    
Senior notes (Level 1), fair value 2,810 2,932
Significant Other Observable Inputs (Level 2)    
Financial liabilities    
Credit facility (Level 2), fair value $ 1,449 $ 1,178