Risk Management and Trading Activities - Netting Arrangements on Physical and Financial Derivative Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross Amounts, Assets	$ 3,698	$ 3,353
Physical Derivative and Financial Instruments, Assets	(3,049)	(2,750)
Cash Collateral, Assets	(49)	(34)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	600	569
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(59)	(51)
Net Amount, Assets	541	518
Gross Amounts, Liabilities	(3,681)	(3,322)
Physical Derivative and Financial Instruments, Liabilities	3,049	2,750
Cash Collateral, Liabilities		5
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(632)	(567)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	59	51
Net Amount, Liabilities	(573)	(516)
Commodity [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts, Assets	3,613	3,253
Physical Derivative and Financial Instruments, Assets	(2,990)	(2,661)
Cash Collateral, Assets	(45)	(34)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	578	558
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(55)	(45)
Net Amount, Assets	523	513
Gross Amounts, Liabilities	(3,668)	(3,312)
Physical Derivative and Financial Instruments, Liabilities	2,990	2,661
Cash Collateral, Liabilities		5
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(678)	(646)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	55	45
Net Amount, Liabilities	(623)	(601)
Other Member]
Derivatives, Fair Value [Line Items]
Gross Amounts, Liabilities	(13)	(10)
Physical Derivative and Financial Instruments, Liabilities	8	8
Cash Collateral, Liabilities
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(5)	(2)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	4	6
Net Amount, Liabilities	(1)	4
Counterparty netting [Member]
Derivatives, Fair Value [Line Items]
Physical Derivative and Financial Instruments, Assets	(51)	(81)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	(51)	(81)
Net Amount, Assets	(51)	(81)
Gross Amounts, Liabilities
Physical Derivative and Financial Instruments, Liabilities	51	81
Cash Collateral, Liabilities
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	51	81
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities
Net Amount, Liabilities	51	81
Interest rate and other [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts, Assets	85	100
Physical Derivative and Financial Instruments, Assets	(8)	(8)
Cash Collateral, Assets	(4)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	73	92
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(4)	(6)
Net Amount, Assets	$ 69	$ 86