Derivative Instruments (Detail 5) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	$ (184)		$ (213)
Derivative Fair Value, Amount Not Offset Against Collateral, Net	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	32		34
Derivative Asset, Fair Value, Gross Asset	737		437
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(687)		(396)
Derivative Liability, Fair Value, Gross Liability	(921)		(650)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	687		396
Margin/Cash (Received) Posted Subject to Master Netting Arrangement	59	[1]	39	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	(157)		(208)
Derivative, Fair Value, Amount Offset Against Collateral, Net	(125)		(174)
Derivative, Collateral, Right to Reclaim Cash	77	[1]	46	[1]
Derivative, Collateral, Obligation to Return Cash	(18)	[1]	(7)	[1]
Commodity Exchange Traded Futures and Swaps Contracts [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	0		0
Derivative Asset, Fair Value, Gross Asset	595		385
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(580)		(379)
Derivative Liability, Fair Value, Gross Liability	(646)		(424)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	580		379
Derivative Liability, Fair Value, Amount Offset Against Collateral	0		0
Derivative, Collateral, Right to Reclaim Cash	66	[1]	45	[1]
Derivative, Collateral, Obligation to Return Cash	(15)	[1]	(6)	[1]
Commodity Forward Contract [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	24		30
Derivative Asset, Fair Value, Gross Asset	134		48
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(107)		(17)
Derivative Liability, Fair Value, Gross Liability	(130)		(26)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	107		17
Derivative Liability, Fair Value, Amount Offset Against Collateral	(12)		(8)
Derivative, Collateral, Right to Reclaim Cash	11	[1]	1	[1]
Derivative, Collateral, Obligation to Return Cash	(3)	[1]	(1)	[1]
Interest Rate Swap [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	(137)		(196)
Derivative Asset, Fair Value, Amount Offset Against Collateral	8		4
Derivative Asset, Fair Value, Gross Asset	8		4
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0		0
Derivative Liability, Fair Value, Gross Liability	(145)		(200)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0		0
Derivative Liability, Fair Value, Amount Offset Against Collateral	(145)		(200)
Derivative, Collateral, Right to Reclaim Cash	0	[1]	0	[1]
Derivative, Collateral, Obligation to Return Cash	$ 0	[1]	$ 0	[1]

[1] Negative balances represent margin deposits posted with us by our counterparties related to our derivative activities that are subject to a master netting arrangement. Positive balances reflect margin deposits and natural gas and power prepayments posted by us with our counterparties related to our derivative activities that are subject to a master netting arrangement. See Note 6 for a further discussion of our collateral.