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Derivative Instruments - Fair Value Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets $ 108 $ 116
Gross derivative liabilities (1,628) (1,592)
Derivative assets netting (46) (69)
Derivative liabilities netting 46 69
Cash collateral from counterparties (1) 0
Cash collateral held by counterparties 173 123
Derivative assets 61 47
Derivative liabilities (1,409) (1,400)
Commodity Derivatives [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 54 85
Gross derivative liabilities (209) (216)
Derivative assets netting (46) (69)
Derivative liabilities netting 46 69
Cash collateral from counterparties (1) 0
Cash collateral held by counterparties 3 6
Derivative assets 7 16
Derivative liabilities (160) (141)
Interest Rate Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 54 31
Gross derivative liabilities (1,419) (1,376)
Derivative assets netting 0 0
Derivative liabilities netting 0 0
Cash collateral from counterparties 0 0
Cash collateral held by counterparties 170 117
Derivative assets 54 31
Derivative liabilities (1,249) (1,259)
Fair Value, Inputs, Level 1 [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 1 2
Gross derivative liabilities (1) (3)
Fair Value, Inputs, Level 1 [Member] | Commodity Derivatives [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 1 2
Gross derivative liabilities (1) (3)
Fair Value, Inputs, Level 1 [Member] | Interest Rate Swap [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 0 0
Gross derivative liabilities 0 0
Fair Value, Inputs, Level 2 [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 107 114
Gross derivative liabilities (1,627) (1,589)
Fair Value, Inputs, Level 2 [Member] | Commodity Derivatives [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 53 83
Gross derivative liabilities (208) (213)
Fair Value, Inputs, Level 2 [Member] | Interest Rate Swap [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 54 31
Gross derivative liabilities (1,419) (1,376)
Fair Value, Inputs, Level 3 [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 0 0
Gross derivative liabilities 0 0
Fair Value, Inputs, Level 3 [Member] | Commodity Derivatives [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 0 0
Gross derivative liabilities 0 0
Fair Value, Inputs, Level 3 [Member] | Interest Rate Swap [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross derivative assets 0 0
Gross derivative liabilities $ 0 $ 0