Derivative Instruments and Hedging Activities (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	$ 117	$ 133
Long-Term, Derivative Assets	242	437
Short-Term, Derivative Liabilities	238	217
Long-Term, Derivative Liabilities	580	468
Commodity contracts | Electric Utility
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	78	87
Long-Term, Derivative Assets	179	367
Derivative Assets, Total	257	454
Short-Term, Derivative Liabilities	231	212
Long-Term, Derivative Liabilities	558	449
Derivative Liabilities, Total	789	661
Net Asset (Liability)	(532)	(207)
Commodity contracts | Electric Utility | Economic hedges
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	89	87
Long-Term, Derivative Assets	200	367
Derivative Assets, Total	289	454
Short-Term, Derivative Liabilities	243	216
Long-Term, Derivative Liabilities	579	449
Derivative Liabilities, Total	822	665
Net Asset (Liability)	(533)	(211)
Commodity contracts | Electric Utility | Netting and Collateral
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	(11)
Long-Term, Derivative Assets	(21)
Derivative Assets, Total	(32)
Short-Term, Derivative Liabilities	(12)	(4)
Long-Term, Derivative Liabilities	(21)
Derivative Liabilities, Total	(33)	(4)
Net Asset (Liability)	1	4
Competitive power generation
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	39	46
Long-Term, Derivative Assets	63	70
Derivative Assets, Total	102	116
Short-Term, Derivative Liabilities	6	6
Long-Term, Derivative Liabilities	23	19
Derivative Liabilities, Total	29	25
Net Asset (Liability)	73	91
Competitive power generation | Trading and Non-trading activities
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	228	315
Long-Term, Derivative Assets	94	107
Derivative Assets, Total	322	422
Short-Term, Derivative Liabilities	163	229
Long-Term, Derivative Liabilities	55	54
Derivative Liabilities, Total	218	283
Net Asset (Liability)	104	139
Competitive power generation | Cash flow hedges
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	27	54
Long-Term, Derivative Assets	2	2
Derivative Assets, Total	29	56
Short-Term, Derivative Liabilities	14	10
Long-Term, Derivative Liabilities	34	25
Derivative Liabilities, Total	48	35
Net Asset (Liability)	(19)	21
Competitive power generation | Economic hedges
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	60	77
Long-Term, Derivative Assets	4	2
Derivative Assets, Total	64	79
Short-Term, Derivative Liabilities	51	71
Long-Term, Derivative Liabilities	1
Derivative Liabilities, Total	52	71
Net Asset (Liability)	12	8
Competitive power generation | Trading activities
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	141	184
Long-Term, Derivative Assets	88	103
Derivative Assets, Total	229	287
Short-Term, Derivative Liabilities	98	148
Long-Term, Derivative Liabilities	20	29
Derivative Liabilities, Total	118	177
Net Asset (Liability)	111	110
Competitive power generation | Netting and Collateral
Fair Value of Derivative Instruments
Short-Term, Derivative Assets	(189)	(269)
Long-Term, Derivative Assets	(31)	(37)
Derivative Assets, Total	(220)	(306)
Short-Term, Derivative Liabilities	(157)	(223)
Long-Term, Derivative Liabilities	(32)	(35)
Derivative Liabilities, Total	(189)	(258)
Net Asset (Liability)	$ (31)	$ (48)