Segment and Significant Customer Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 1,367	$ 1,996	$ 879	$ 1,236	$ 1,459	$ 2,209	$ 1,633	$ 1,499	$ 5,478		$ 6,800		$ 6,545
Intersegment revenues									0		0		0
Total operating revenues									5,478		6,800		6,545
Commodity Margin									2,538	[1],[2]	2,474	[1],[2]	2,391	[1],[2]
Add: Mark-to-market commodity activity, net and other									(84)	[3]	(33)	[3]	171
Plant operating expense									922		904		868
Depreciation and amortization expense									562		550		570
Sales, general and other administrative expense									140		131		151
Other operating expenses									78		77		91
Impairment losses													116
Gain (Loss) on Disposition of Assets									(222)		0		(119)
(Income) from unconsolidated investments in power plants									(28)		(21)		(16)
Income from operations	295	705	(193)	195	196	403	183	18	1,002		800		901
Interest expense, net of interest income									725		751		802
Loss on interest rate derivatives									14		145		223
Debt extinguishment costs and other (income) expense, net									45		115		106
Income (loss) before income taxes and discontinued operations									218		(211)		(230)
Commodity Margin Riverside Energy Center									73		70		73
Commodity Margin Broad River Energy Center									52		51		55
Lease levelization									1		12
Contract amortization									14		8
Number of significant customers									1
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									1,668		2,372		2,525
Intersegment revenues									10		12		12
Total operating revenues									1,678		2,384		2,537
Commodity Margin									994	[1],[2]	1,061	[1],[2]	1,080	[1],[2]
Add: Mark-to-market commodity activity, net and other									(93)	[3]	113	[3]	69
Plant operating expense									368		380		351
Depreciation and amortization expense									203		192		207
Sales, general and other administrative expense									36		43		55
Other operating expenses									42		41		59
Impairment losses													97
Gain (Loss) on Disposition of Assets									0				0
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									252		518		380
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									1,857		2,306		2,162
Intersegment revenues									61		23		22
Total operating revenues									1,918		2,329		2,184
Commodity Margin									570	[1],[2]	469	[1],[2]	504	[1],[2]
Add: Mark-to-market commodity activity, net and other									87	[3]	(102)	[3]	89
Plant operating expense									247		235		285
Depreciation and amortization expense									142		135		150
Sales, general and other administrative expense									47		43		38
Other operating expenses									5		3		2
Impairment losses													0
Gain (Loss) on Disposition of Assets									0				(119)
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									216		(49)		237
North [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									1,280		1,336		978
Intersegment revenues									14		7		6
Total operating revenues									1,294		1,343		984
Commodity Margin									729	[1],[2]	704	[1],[2]	535	[1],[2]
Add: Mark-to-market commodity activity, net and other									(14)	[3]	(13)	[3]	21
Plant operating expense									206		177		138
Depreciation and amortization expense									134		138		111
Sales, general and other administrative expense									28		24		45
Other operating expenses									29		30		28
Impairment losses													0
Gain (Loss) on Disposition of Assets									(7)				0
(Income) from unconsolidated investments in power plants									(28)		(21)		(16)
Income from operations									353		343		250
Southeast [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									673		786		880
Intersegment revenues									80		135		138
Total operating revenues									753		921		1,018
Commodity Margin									245	[1],[2]	240	[1],[2]	272	[1],[2]
Add: Mark-to-market commodity activity, net and other									(33)	[3]	1	[3]	22
Plant operating expense									131		141		123
Depreciation and amortization expense									85		90		109
Sales, general and other administrative expense									29		22		12
Other operating expenses									5		5		4
Impairment losses													19
Gain (Loss) on Disposition of Assets									(215)				0
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									177		(17)		27
Consolidation and Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers									0		0		0
Intersegment revenues									(165)		(177)		(178)
Total operating revenues									(165)		(177)		(178)
Commodity Margin									0	[1],[2]	0	[1],[2]	0	[1],[2]
Add: Mark-to-market commodity activity, net and other									(31)	[3]	(32)	[3]	(30)
Plant operating expense									(30)		(29)		(29)
Depreciation and amortization expense									(2)		(5)		(7)
Sales, general and other administrative expense									0		(1)		1
Other operating expenses									(3)		(2)		(2)
Impairment losses													0
Gain (Loss) on Disposition of Assets									0				0
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									4		5		7
PJM Settlement, Inc. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Major customer revenue									713		742
Major customer receivables									$ 37		$ 28

[1] Our North segment includes Commodity Margin related to Riverside Energy Center, LLC of $73 million , $70 million and $73 million for the years ended December 31, 2012, 2011 and 2010, respectively.

[2] Our Southeast segment includes Commodity Margin related to Broad River of $52 million, $51 million and $55 million for the years ended December 31, 2012, 2011 and 2010, respectively.

[3] Includes $1 million and $12 million of lease levelization and $14 million and $8 million of amortization expense for the years ended December 31, 2012 and 2011, respectively, related to contracts that became effective in 2011.