Derivative Instruments - Fair Value Table (Detail) - Not Designated as Hedging Instrument [Member] - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		$ 351	$ 526
Gross derivative liabilities		(1,758)	(1,722)
Derivative assets netting	[1]	(116)	(178)
Derivative liabilities netting	[1]	116	178
Cash collateral from counterparties		(1)	(8)
Cash collateral held by counterparties		421	58
Derivative assets		234	340
Derivative liabilities		(1,221)	(1,486)
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		329	470
Gross derivative liabilities		(118)	(180)
Derivative assets netting	[1]	(116)	(178)
Derivative liabilities netting	[1]	116	178
Cash collateral from counterparties		(1)	(8)
Cash collateral held by counterparties		0	0
Derivative assets		212	284
Derivative liabilities		(2)	(2)
Interest-Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		22	56
Gross derivative liabilities		(1,640)	(1,542)
Derivative assets netting	[1]	0	0
Derivative liabilities netting	[1]	0	0
Cash collateral from counterparties		0	0
Cash collateral held by counterparties		421	58
Derivative assets		22	56
Derivative liabilities		(1,219)	(1,484)
Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets		1	10
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		350	516
Gross derivative liabilities		(1,758)	(1,721)
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		1	10
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		328	460
Gross derivative liabilities		(118)	(179)
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		22	56
Gross derivative liabilities		(1,640)	(1,542)
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.