Derivative Instruments (Detail 5) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative, Fair Value, Gross Amount Not Offset Against Collateral, Net	$ (129)		$ (144)
Derivative Fair Value, Amount Not Offset Against Collateral, Net	0		0
Derivative Asset, Fair Value, Amount Offset Against Collateral	34		56
Derivative Asset, Fair Value, Gross Asset	829		550
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(747)		(480)
Derivative Liability, Fair Value, Gross Liability	(958)		(694)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	747		480
Margin/Cash (Received) Posted Subject to Master Netting Arrangement	(26)	[1]	62	[1]
Derivative Liability, Fair Value, Amount Offset Against Collateral	(189)		(138)
Derivative, Fair Value, Amount Offset Against Collateral, Net	(155)		(82)
Derivative, Collateral, Right to Reclaim Cash	22	[1]	76	[1]
Derivative, Collateral, Obligation to Return Cash	(48)	[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	717		434
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(669)		(420)
Derivative Liability, Fair Value, Gross Liability	(684)		(495)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	669		420
Derivative Liability, Fair Value, Amount Offset Against Collateral	0		0
Derivative, Collateral, Right to Reclaim Cash	15	[1]	75	[1]
Derivative, Collateral, Obligation to Return Cash	(48)	[1]	(14)	[1]
Commodity Forward Contract [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Amount Offset Against Collateral	27		47
Derivative Asset, Fair Value, Gross Asset	105		107
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	(78)		(60)
Derivative Liability, Fair Value, Gross Liability	(148)		(70)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	78		60
Derivative Liability, Fair Value, Amount Offset Against Collateral	(63)		(9)
Derivative, Collateral, Right to Reclaim Cash	7	[1]	1	[1]
Derivative, Collateral, Obligation to Return Cash	0	[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	(119)		(120)
Derivative Asset, Fair Value, Amount Offset Against Collateral	7		9
Derivative Asset, Fair Value, Gross Asset	7		9
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	0		0
Derivative Liability, Fair Value, Gross Liability	(126)		(129)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0		0
Derivative Liability, Fair Value, Amount Offset Against Collateral	(126)		(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 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.