Fair Value of Derivative Financial Assets and Liabilities by Level within Fair Value Hierarchy Table (Detail) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	$ 599		$ 801
Gross derivative liabilities	(1,287)		(724)
Derivative assets netting	(95)	[1]	(446)	[1]
Derivative liabilities netting	95	[1]	446	[1]
Cash collateral from counterparties	(5)		(15)
Cash collateral held by counterparties	115		15
Derivative financial assets	499		340
Derivative financial liabilities	(1,077)		(263)
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	(3)		(2)
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	(3)		(2)
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross derivative assets	597		798
Gross derivative liabilities	(1,284)		(722)
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	466		557
Gross derivative liabilities	(88)		(333)
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	131		241
Gross derivative liabilities	(24)		(154)
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	(1,172)		(235)
Financial Institutions [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(84)	[1]	(298)	[1]
Derivative liabilities netting	84	[1]	298	[1]
Cash collateral from counterparties	(5)		(15)
Cash collateral held by counterparties	5
Derivative financial assets	379		247
Derivative financial liabilities	(2)		(37)
Other Counterparties [Member] | Commodity Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets netting	(11)	[1]	(148)	[1]
Derivative liabilities netting	11	[1]	148	[1]
Derivative financial assets	120		93
Derivative financial liabilities	(13)		(6)
Interest Rate Contract and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral held by counterparties	110		15
Derivative financial liabilities	$ (1,062)		$ (220)

[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.