Derivative Instruments - Fair Value Table (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 218		$ 506
Gross derivative liabilities	(1,377)		(1,419)
Derivative assets netting	(180)	[1]	(211)	[1]
Derivative liabilities netting	180	[1]	211	[1]
Cash collateral from counterparties	(1)
Cash collateral held by counterparties	4		1
Derivative financial assets	37		295
Derivative financial liabilities	(1,193)		(1,207)
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	218		506
Gross derivative liabilities	(276)		(225)
Interest-Rate and Other Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative financial liabilities	(1,101)		(1,194)
Financial Institutions [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(178)	[1]	(206)	[1]
Derivative liabilities netting	178	[1]	206	[1]
Cash collateral from counterparties	(1)
Cash collateral held by counterparties	4		1
Derivative financial assets	13		253
Derivative financial liabilities	(71)		(1)
Other Counterparties [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(2)	[1]	(5)	[1]
Derivative liabilities netting	2	[1]	5	[1]
Derivative financial assets	24		42
Derivative financial liabilities	(21)		(12)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	3		6
Gross derivative liabilities	(3)		(6)
Fair Value, Inputs, Level 1 [Member] | Financial Institutions [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	3		6
Gross derivative liabilities	(3)		(6)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	215		500
Gross derivative liabilities	(1,374)		(1,413)
Fair Value, Inputs, Level 2 [Member] | Interest-Rate and Other Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities	(1,101)		(1,194)
Fair Value, Inputs, Level 2 [Member] | Financial Institutions [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	189		453
Gross derivative liabilities	(250)		(202)
Fair Value, Inputs, Level 2 [Member] | Other Counterparties [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	26		47
Gross derivative liabilities	$ (23)		$ (17)

[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.