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