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Debt - Summary of Carrying Amounts and Estimated Fair Values (Detail) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Debt, Carrying Value $ 8,544 $ 8,717
Debt issuance costs 56 61
Debt, Fair Value 9,183 8,330
Commercial Paper [Member]    
Debt Instrument [Line Items]    
Debt, Carrying Value 0 0
Debt, Fair Value 0 0
Notes and Debentures [Member]    
Debt Instrument [Line Items]    
Debt, Carrying Value 8,544 8,717
Debt, Fair Value $ 9,183 $ 8,330