Derivative Instruments - Fair Value Table (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 506		$ 1,080
Gross derivative liabilities	(1,419)		(1,617)
Derivative assets netting	(211)	[1]	(374)	[1]
Derivative liabilities netting	211	[1]	374	[1]
Cash collateral from counterparties			(52)
Cash collateral held by counterparties	1		137
Derivative financial assets	295		654
Derivative financial liabilities	(1,207)		(1,106)
Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	506		1,080
Gross derivative liabilities	(225)		(418)
Interest-Rate and Other Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities	(1,194)		(1,199)
Cash collateral held by counterparties			130
Derivative financial liabilities	(1,194)		(1,069)
Financial Institutions [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(206)	[1]	(361)	[1]
Derivative liabilities netting	206	[1]	361	[1]
Cash collateral from counterparties			(52)
Cash collateral held by counterparties	1		7
Derivative financial assets	253		537
Derivative financial liabilities	(1)		(11)
Other Counterparties [Member] | Commodity Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(5)	[1]	(13)	[1]
Derivative liabilities netting	5	[1]	13	[1]
Derivative financial assets	42		117
Derivative financial liabilities	(12)		(26)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	6		3
Gross derivative liabilities	(6)		(4)
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	6		3
Gross derivative liabilities	(6)		(4)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	500		1,077
Gross derivative liabilities	(1,413)		(1,613)
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	(1,194)		(1,199)
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	453		947
Gross derivative liabilities	(202)		(375)
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	47		130
Gross derivative liabilities	$ (17)		$ (39)

[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.