Derivative Instruments - Fair Value of Derivative Financial Assets and Liabilities by Level within Fair Value Hierarchy Table (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 579		$ 1,080
Gross derivative liabilities	(1,655)		(1,617)
Derivative assets netting	(280)	[1]	(374)	[1]
Derivative liabilities netting	280	[1]	374	[1]
Cash collateral from counterparties	(10)		(52)
Cash collateral held by counterparties	10		137
Derivative financial assets	289		654
Derivative financial liabilities	(1,365)		(1,106)
Interest Rate Contract and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral held by counterparties			130
Derivative financial liabilities	(1,351)		(1,069)
Financial Institutions [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(272)	[1]	(361)	[1]
Derivative liabilities netting	272	[1]	361	[1]
Cash collateral from counterparties	(10)		(52)
Cash collateral held by counterparties	10		7
Derivative financial assets	237		537
Derivative financial liabilities	2		(11)
Other Counterparties [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(8)	[1]	(13)	[1]
Derivative liabilities netting	8	[1]	13	[1]
Derivative financial assets	52		117
Derivative financial liabilities	(16)		(26)
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	2		3
Gross derivative liabilities	(2)		(4)
Fair Value, Inputs, Level 1 [Member] | Financial Institutions [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	2		3
Gross derivative liabilities	(2)		(4)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	577		1,077
Gross derivative liabilities	(1,653)		(1,613)
Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative liabilities	(1,351)		(1,199)
Fair Value, Inputs, Level 2 [Member] | Financial Institutions [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	517		947
Gross derivative liabilities	(278)		(375)
Fair Value, Inputs, Level 2 [Member] | Other Counterparties [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	60		130
Gross derivative liabilities	$ (24)		$ (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.