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Debt - Summary of Carrying Amounts and Estimated Fair Values (Detail) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Commercial paper, Carrying Amount   $ 1,570
Total Debt, Carrying Amount $ 8,777 11,245
Total Debt, Fair Value 8,330 11,514
Commercial Paper [Member]    
Debt Instrument [Line Items]    
Total Debt, Fair Value   1,570
Notes and Debentures [Member]    
Debt Instrument [Line Items]    
Notes and debentures, Carrying Amount 8,778 9,675
Total Debt, Fair Value $ 8,330 $ 9,944