Derivative Instruments - Fair Value Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions	Sep. 30, 2018		Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 112		$ 108
Gross derivative liabilities	(1,604)		(1,628)
Derivative assets Netting	(35)	[1]	(46)
Derivative liabilities Netting	35	[1]	46
Derivative assets, Collateral	0		(1)
Derivative liabilities, Collateral	55		173
Derivative assets	77		61
Derivative liabilities	(1,514)		(1,409)
Commodity derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	37		54
Gross derivative liabilities	(496)		(209)
Derivative assets Netting	(35)	[1]	(46)
Derivative liabilities Netting	35	[1]	46
Derivative assets, Collateral	0		(1)
Derivative liabilities, Collateral	6		3
Derivative assets	2		7
Derivative liabilities	(455)		(160)
Interest-rate derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	75		54
Gross derivative liabilities	(1,108)		(1,419)
Derivative assets Netting	0	[1]	0
Derivative liabilities Netting	0	[1]	0
Derivative assets, Collateral	0		0
Derivative liabilities, Collateral	49		170
Derivative assets	75		54
Derivative liabilities	(1,059)		(1,249)
Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		1
Gross derivative liabilities	0		(1)
Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	112		107
Gross derivative liabilities	(1,604)		(1,627)
Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Recurring [Member] | Commodity derivatives [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		1
Gross derivative liabilities	0		(1)
Recurring [Member] | Commodity derivatives [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	37		53
Gross derivative liabilities	(496)		(208)
Recurring [Member] | Commodity derivatives [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Recurring [Member] | Interest-rate derivatives [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	0		0
Recurring [Member] | Interest-rate derivatives [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	75		54
Gross derivative liabilities	(1,108)		(1,419)
Recurring [Member] | Interest-rate derivatives [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	0		0
Gross derivative liabilities	$ 0		$ 0

[1] Represents the impact of netting commodity derivative assets and liabilities with counterparties where the Company has the contractual right and intends to net settle.