Derivative Instruments (Detail 5) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 2,497		$ 550
Derivative Asset, Not Offset, Policy Election Deduction	(2,087)		(480)
Derivative, Collateral, Obligation to Return Cash	(269)	[1]	(14)	[1]
Derivative Asset, Fair Value, Amount Offset Against Collateral	141		56
Derivative Liability, Fair Value, Gross Liability	(2,226)		(694)
Derivative Liability, Not Offset, Policy Election Deduction	2,087		480
Derivative, Collateral, Right to Reclaim Cash	15	[1]	76	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	(124)		(138)
Net derivative assets (liabilities)	271		(144)
Derivative Fair Value, Amount Not Offset Against Collateral, Net	0		0
Derivative, Collateral, Right to Reclaim Cash, Net	(254)	[1]	62	[1]
Derivative, Fair Value, Amount Offset Against Collateral, Net	17		(82)
Commodity Exchange Traded Futures and Swaps Contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	2,134		434
Derivative Asset, Not Offset, Policy Election Deduction	(1,865)		(420)
Derivative, Collateral, Obligation to Return Cash	(269)	[1]	(14)	[1]
Derivative Asset, Fair Value, Amount Offset Against Collateral	0		0
Derivative Liability, Fair Value, Gross Liability	(1,870)		(495)
Derivative Liability, Not Offset, Policy Election Deduction	1,865		420
Derivative, Collateral, Right to Reclaim Cash	5	[1]	75	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	0		0
Commodity Forward Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	359		107
Derivative Asset, Not Offset, Policy Election Deduction	(222)		(60)
Derivative, Collateral, Obligation to Return Cash	0	[1]	0	[1]
Derivative Asset, Fair Value, Amount Offset Against Collateral	137		47
Derivative Liability, Fair Value, Gross Liability	(242)		(70)
Derivative Liability, Not Offset, Policy Election Deduction	222		60
Derivative, Collateral, Right to Reclaim Cash	10	[1]	1	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	(10)		(9)
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	4		9
Derivative Asset, Not Offset, Policy Election Deduction	0		0
Derivative, Collateral, Obligation to Return Cash	0	[1]	0	[1]
Derivative Asset, Fair Value, Amount Offset Against Collateral	4		9
Derivative Liability, Fair Value, Gross Liability	(114)		(129)
Derivative Liability, Not Offset, Policy Election Deduction	0		0
Derivative, Collateral, Right to Reclaim Cash	0	[1]	0	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	(114)		(129)
Net derivative assets (liabilities)	$ (110)		$ (120)

[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 9 for a further discussion of our collateral.