Segment Information (Details) (USD $) In Millions	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Information [Line Items]
Revenues from external customers	$ 2,209	$ 2,130	$ 5,341	$ 5,074
Intersegment revenues
Total operating revenues	2,209	2,130	5,341	5,074
Commodity Margin	825	852	1,921	1,815
Mark-to-Market Commodity Activity, Net and Other Revenue	(20)	134	(57)	237
Plant operating expense	212	199	711	630
Depreciation and amortization expense	143	152	405	423
Sales, general and other administrative expense	33	41	99	113
Other operating expense	22	23	64	75
Impairment losses		19		19
(Income) from unconsolidated investments in power plants	5	1	12	14
Income from operations	403	554	604	812
Interest expense, net of interest income	190	228	568	627
(Gain) loss on interest rate derivatives, net	3	84	149	87
Debt Extinguishment Costs and Other (Income) Expense, Net		23	108	36
Loss before income taxes and discontinued operations	210	219	(221)	62
Lease levelization	11		15
Contract amortization	4		5
RGGI Compliance and Other Environmental Costs	3	1	7	6
West
Segment Information [Line Items]
Revenues from external customers	688	716	1,753	1,906
Intersegment revenues	3	2	7	7
Total operating revenues	691	718	1,760	1,913
Commodity Margin	329	338	798	809
Mark-to-Market Commodity Activity, Net and Other Revenue	20	42	36	60
Plant operating expense	94	86	297	264
Depreciation and amortization expense	52	52	140	155
Sales, general and other administrative expense	10	10	29	36
Other operating expense	11	14	30	43
(Income) from unconsolidated investments in power plants
Income from operations	182	218	338	371
Texas
Segment Information [Line Items]
Revenues from external customers	843	670	1,939	1,749
Intersegment revenues	3	6	13	16
Total operating revenues	846	676	1,952	1,765
Commodity Margin	162	165	357	400
Mark-to-Market Commodity Activity, Net and Other Revenue	(21)	62	(54)	148
Plant operating expense	50	55	193	217
Depreciation and amortization expense	34	37	99	113
Sales, general and other administrative expense	10	13	33	29
Other operating expense	(1)		2	2
(Income) from unconsolidated investments in power plants
Income from operations	48	122	(24)	187
North
Segment Information [Line Items]
Revenues from external customers	430	468	1,025	725
Intersegment revenues	(8)	2	5	4
Total operating revenues	422	470	1,030	729
Commodity Margin	259	259	578	390
Mark-to-Market Commodity Activity, Net and Other Revenue	(11)	18	(12)	18
Plant operating expense	44	38	136	83
Depreciation and amortization expense	36	37	102	76
Sales, general and other administrative expense	7	12	19	37
Other operating expense	7	6	23	21
(Income) from unconsolidated investments in power plants	(5)	(1)	(12)	(14)
Income from operations	159	185	298	205
Southeast
Segment Information [Line Items]
Revenues from external customers	248	276	624	694
Intersegment revenues	31	53	116	97
Total operating revenues	279	329	740	791
Commodity Margin	75	90	188	216
Mark-to-Market Commodity Activity, Net and Other Revenue		18	(4)	31
Plant operating expense	33	28	107	87
Depreciation and amortization expense	22	28	67	84
Sales, general and other administrative expense	7	5	18	11
Other operating expense			3	2
Impairment losses		19		19
(Income) from unconsolidated investments in power plants
Income from operations	13	28	(11)	44
Consolidation and Elimination
Segment Information [Line Items]
Revenues from external customers
Intersegment revenues	(29)	(63)	(141)	(124)
Total operating revenues	(29)	(63)	(141)	(124)
Commodity Margin
Mark-to-Market Commodity Activity, Net and Other Revenue	(8)	(6)	(23)	(20)
Plant operating expense	(9)	(8)	(22)	(21)
Depreciation and amortization expense	(1)	(2)	(3)	(5)
Sales, general and other administrative expense	(1)	1
Other operating expense	2	2	(1)	1
(Income) from unconsolidated investments in power plants
Income from operations	$ 1	$ 1	$ 3	$ 5

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