Derivative Instruments (Detail 5) (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 2,011	$ 2,497
Derivative Asset, Not Offset, Policy Election Deduction		(1,804)	(2,087)
Derivative, Collateral, Obligation to Return Cash	[1]	(137)	(269)
Derivative Asset, Fair Value, Amount Offset Against Collateral		70	141
Derivative Liability, Fair Value, Gross Liability		(2,207)	(2,226)
Derivative Liability, Not Offset, Policy Election Deduction		1,804	2,087
Derivative, Collateral, Right to Reclaim Cash	[1]	5	15
Derivative Liability, Fair Value, Amount Offset Against Collateral		(398)	(124)
Net derivative assets (liabilities)		(196)	271
Derivative Fair Value, Amount Not Offset Against Collateral, Net		0	0
Derivative, Collateral, Right to Reclaim Cash, Net	[1]	(132)	(254)
Derivative, Fair Value, Amount Offset Against Collateral, Net		(328)	17
Commodity Exchange Traded Futures and Swaps Contracts [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		1,736	2,134
Derivative Asset, Not Offset, Policy Election Deduction		(1,602)	(1,865)
Derivative, Collateral, Obligation to Return Cash	[1]	(134)	(269)
Derivative Asset, Fair Value, Amount Offset Against Collateral		0	0
Derivative Liability, Fair Value, Gross Liability		(1,604)	(1,870)
Derivative Liability, Not Offset, Policy Election Deduction		1,602	1,865
Derivative, Collateral, Right to Reclaim Cash	[1]	2	5
Derivative Liability, Fair Value, Amount Offset Against Collateral		0	0
Commodity Forward Contract [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		274	359
Derivative Asset, Not Offset, Policy Election Deduction		(202)	(222)
Derivative, Collateral, Obligation to Return Cash	[1]	(3)	0
Derivative Asset, Fair Value, Amount Offset Against Collateral		69	137
Derivative Liability, Fair Value, Gross Liability		(513)	(242)
Derivative Liability, Not Offset, Policy Election Deduction		202	222
Derivative, Collateral, Right to Reclaim Cash	[1]	3	10
Derivative Liability, Fair Value, Amount Offset Against Collateral		(308)	(10)
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		1	4
Derivative Asset, Not Offset, Policy Election Deduction		0	0
Derivative, Collateral, Obligation to Return Cash	[1]	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral		1	4
Derivative Liability, Fair Value, Gross Liability		(90)	(114)
Derivative Liability, Not Offset, Policy Election Deduction		0	0
Derivative, Collateral, Right to Reclaim Cash	[1]	0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral		(90)	(114)
Net derivative assets (liabilities)		$ (89)	$ (110)

[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.