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Fair Value Measurements - Recurring - Financial Instruments by Valuation Hierarchy (Details) - Fair Value, Measurements, Recurring [Member] - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset $ 2 $ 0
Derivative liability 43 60
Commodity contracts | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset 0 0
Derivative liability 0 0
Commodity contracts | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset 2 0
Derivative liability 0 6
Embedded derivatives in commodity contracts | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative asset 0 0
Derivative liability $ 43 $ 54