Derivative Instruments (Detail 5) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	$ 28		$ (144)
Derivative Fair Value, Amount Not Offset Against Collateral, Net	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	100		56
Derivative Asset, Fair Value, Gross Asset	982		550
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(749)		(480)
Derivative Liability, Fair Value, Gross Liability	(954)		(694)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	749		480
Margin/Cash (Received) Posted Subject to Master Netting Arrangement	(72)	[1]	62	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	(144)		(138)
Derivative, Fair Value, Amount Offset Against Collateral, Net	(44)		(82)
Derivative, Collateral, Right to Reclaim Cash	61	[1]	76	[1]
Derivative, Collateral, Obligation to Return Cash	(133)	[1]	(14)	[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	681		434
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(590)		(420)
Derivative Liability, Fair Value, Gross Liability	(640)		(495)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	590		420
Derivative Liability, Fair Value, Amount Offset Against Collateral	0		0
Derivative, Collateral, Right to Reclaim Cash	50	[1]	75	[1]
Derivative, Collateral, Obligation to Return Cash	(91)	[1]	(14)	[1]
Commodity Forward Contract [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	95		47
Derivative Asset, Fair Value, Gross Asset	296		107
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(159)		(60)
Derivative Liability, Fair Value, Gross Liability	(184)		(70)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	159		60
Derivative Liability, Fair Value, Amount Offset Against Collateral	(14)		(9)
Derivative, Collateral, Right to Reclaim Cash	11	[1]	1	[1]
Derivative, Collateral, Obligation to Return Cash	(42)	[1]	0	[1]
Interest Rate Swap [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	(125)		(120)
Derivative Asset, Fair Value, Amount Offset Against Collateral	5		9
Derivative Asset, Fair Value, Gross Asset	5		9
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0		0
Derivative Liability, Fair Value, Gross Liability	(130)		(129)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0		0
Derivative Liability, Fair Value, Amount Offset Against Collateral	(130)		(129)
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 7 for a further discussion of our collateral.