Risk Management and Trading Activities - Netting Arrangements on Physical and Financial Derivative Assets and (Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross Amounts, Assets	$ 3,248	$ 3,353
Physical Derivative and Financial Instruments, Assets	(2,621)	(2,750)
Cash Collateral, Assets	(23)	(34)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	604	569
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(32)	(51)
Net Amounts, Assets	572	518
Gross Amounts, Liabilities	(3,332)	(3,322)
Physical Derivative and Financial Instruments, Liabilities	2,621	2,750
Cash Collateral, Liabilities	99	5
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(612)	(567)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	32	51
Net Amounts, Liabilities	(580)	(516)
Commodity [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts, Assets	3,186	3,253
Physical Derivative and Financial Instruments, Assets	(2,530)	(2,661)
Cash Collateral, Assets	(19)	(34)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	637	558
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(30)	(45)
Net Amounts, Assets	607	513
Gross Amounts, Liabilities	(3,320)	(3,312)
Physical Derivative and Financial Instruments, Liabilities	2,530	2,661
Cash Collateral, Liabilities	99	5
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(691)	(646)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	30	45
Net Amounts, Liabilities	(661)	(601)
Other [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts, Liabilities	(12)	(10)
Physical Derivative and Financial Instruments, Liabilities	11	8
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	(1)	(2)
Gross Amounts Not Offset in the Consolidated Balance Sheet, Liabilities	2	6
Net Amounts, Liabilities	1	4
Counterparty Netting Other [Member]
Derivatives, Fair Value [Line Items]
Physical Derivative and Financial Instruments, Assets	(80)	(81)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	(80)	(81)
Net Amounts, Assets	(80)	(81)
Physical Derivative and Financial Instruments, Liabilities	80	81
Net Amounts Presented in the Consolidated Balance Sheet, Liabilities	80	81
Net Amounts, Liabilities	80	81
Interest rate and other [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts, Assets	62	100
Physical Derivative and Financial Instruments, Assets	(11)	(8)
Cash Collateral, Assets	(4)
Net Amounts Presented in the Consolidated Balance Sheet, Assets	47	92
Gross Amounts Not Offset in the Consolidated Balance Sheet, Assets	(2)	(6)
Net Amounts, Assets	$ 45	$ 86