Segment Information (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Information [Line Items]
Revenues from external customers	$ 1,633	$ 1,430	$ 3,132	$ 2,944
Intersegment revenues
Total operating revenues	1,633	1,430	3,132	2,944
Commodity Margin	602	533	1,091	963
Mark-to-Market Commodity Activity, Net and Other Revenue	29	(12)	(32)	103
Plant operating expense	261	213	499	431
Depreciation and amortization expense	131	135	262	271
Sales, general and other administrative expense	34	50	66	72
Other operating expense	22	26	42	52
(Income) from unconsolidated investments in power plants	(2)	6	7	13
Income from operations	183	108	201	258
Interest expense, net of interest income	190	220	378	399
(Gain) loss on interest rate derivatives, net	37	(8)	146	3
Debt Extinguishment Costs and Other (Income) Expense, Net	8	8	108	13
Loss before income taxes and discontinued operations	(52)	(112)	(431)	(157)
RGGI Compliance and Other Environmental Costs	2	5	4	5
West
Segment Information [Line Items]
Revenues from external customers	466	525	1,065	1,190
Intersegment revenues	1	1	4	5
Total operating revenues	467	526	1,069	1,195
Commodity Margin	236	258	469	471
Mark-to-Market Commodity Activity, Net and Other Revenue	11	10	16	18
Plant operating expense	116	88	203	178
Depreciation and amortization expense	42	50	88	103
Sales, general and other administrative expense	8	11	19	26
Other operating expense	11	12	19	29
(Income) from unconsolidated investments in power plants
Income from operations	70	107	156	153
Texas
Segment Information [Line Items]
Revenues from external customers	646	552	1,096	1,079
Intersegment revenues	5	6	10	10
Total operating revenues	651	558	1,106	1,089
Commodity Margin	128	128	195	235
Mark-to-Market Commodity Activity, Net and Other Revenue	27	(10)	(33)	86
Plant operating expense	63	78	143	162
Depreciation and amortization expense	35	40	65	76
Sales, general and other administrative expense	13	16	23	16
Other operating expense	3	(5)	3	2
(Income) from unconsolidated investments in power plants
Income from operations	41	(11)	(72)	65
North
Segment Information [Line Items]
Revenues from external customers	324	134	595	257
Intersegment revenues	5	1	13	2
Total operating revenues	329	135	608	259
Commodity Margin	179	79	314	131
Mark-to-Market Commodity Activity, Net and Other Revenue		3	4
Plant operating expense	47	23	92	45
Depreciation and amortization expense	33	19	66	39
Sales, general and other administrative expense	6	22	12	25
Other operating expense	9	7	16	15
(Income) from unconsolidated investments in power plants	2	(6)	(7)	(13)
Income from operations	82	17	139	20
Southeast
Segment Information [Line Items]
Revenues from external customers	197	219	376	418
Intersegment revenues	40	21	85	44
Total operating revenues	237	240	461	462
Commodity Margin	59	68	113	126
Mark-to-Market Commodity Activity, Net and Other Revenue		(9)	(4)	13
Plant operating expense	41	31	74	59
Depreciation and amortization expense	22	27	45	56
Sales, general and other administrative expense	6	2	11	6
Other operating expense	2	(1)	3	2
(Income) from unconsolidated investments in power plants
Income from operations	(12)		(24)	16
Consolidation and Elimination
Segment Information [Line Items]
Revenues from external customers
Intersegment revenues	(51)	(29)	(112)	(61)
Total operating revenues	(51)	(29)	(112)	(61)
Commodity Margin
Mark-to-Market Commodity Activity, Net and Other Revenue	(9)	(6)	(15)	(14)
Plant operating expense	(6)	(7)	(13)	(13)
Depreciation and amortization expense	(1)	(1)	(2)	(3)
Sales, general and other administrative expense	1	(1)	1	(1)
Other operating expense	(5)	8	(3)	(1)
(Income) from unconsolidated investments in power plants
Income from operations	$ 2	$ (5)	$ 2	$ 4