Derivative Instruments (Detail 3) (Details) - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 1,600	$ 2,011
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		(1,469)	(1,804)
Derivative, Collateral, Obligation to Return Cash	[1]	(81)	(137)
Derivative Liability, Fair Value, Gross Liability		(1,872)	(2,207)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		1,469	1,804
Derivative, Collateral, Right to Reclaim Cash	[1]	23	5
Derivative, Fair Value, Net		(272)	(196)
Derivative Fair Value, Amount Not Offset Against Collateral, Net		0	0
Margin/Cash (Received) Posted Subject to Master Netting Arrangement	[1]	(58)	(132)
Derivative Asset, Fair Value, Amount Offset Against Collateral		50	70
Derivative Liability, Fair Value, Amount Offset Against Collateral		(380)	(398)
Derivative, Fair Value, Amount Offset Against Collateral, Net		(330)	(328)
Commodity Exchange Traded Futures and Swaps Contracts [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Gross Asset		1,316	1,736
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		(1,237)	(1,602)
Derivative, Collateral, Obligation to Return Cash	[1]	(79)	(134)
Derivative Liability, Fair Value, Gross Liability		(1,248)	(1,604)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		1,237	1,602
Derivative, Collateral, Right to Reclaim Cash	[1]	11	2
Derivative Asset, Fair Value, Amount Offset Against Collateral		0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral		0	0
Commodity Forward Contract [Member]
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Gross Asset		275	274
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		(232)	(202)
Derivative, Collateral, Obligation to Return Cash	[1]	(2)	(3)
Derivative Liability, Fair Value, Gross Liability		(528)	(513)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		232	202
Derivative, Collateral, Right to Reclaim Cash	[1]	12	3
Derivative Asset, Fair Value, Amount Offset Against Collateral		41	69
Derivative Liability, Fair Value, Amount Offset Against Collateral		(284)	(308)
Interest Rate Hedging Instruments
Derivative Instruments Subject to Master Netting Arrangement [Line Items]
Derivative Asset, Fair Value, Gross Asset		9	1
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0	0
Derivative, Collateral, Obligation to Return Cash	[1]	0	0
Derivative Liability, Fair Value, Gross Liability		(96)	(90)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		0	0
Derivative, Collateral, Right to Reclaim Cash	[1]	0	0
Derivative, Fair Value, Net		(87)	(89)
Derivative Asset, Fair Value, Amount Offset Against Collateral		9	1
Derivative Liability, Fair Value, Amount Offset Against Collateral		$ (96)	$ (90)

[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.