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Derivatives - Classification of Gross Fair Values of Derivative Instruments, Excluding Cash Collateral (Detail) - Commodity derivatives - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Other current assets    
Derivatives, Fair Value [Line Items]    
Asset $ 21 $ 127
Liability 60 126
Other current liabilities(a)    
Derivatives, Fair Value [Line Items]    
Asset 0 0
Liability [1] 12 14
Deferred credits and other liabilities    
Derivatives, Fair Value [Line Items]    
Asset 0 0
Liability [1] $ 48 $ 52
[1] Includes embedded derivatives.