Segment and Significant Customer Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer									$ 6,301		$ 5,478		$ 6,800
Intersegment revenue									0		0		0
Operating revenues	1,438	2,050	1,572	1,241	1,367	1,996	879	1,236	6,301		5,478		6,800
Commodity Margin									2,568		2,538	[1],[2]	2,474	[1],[2]
Add: Mark-to-market commodity activity, net and other									(3)	[3]	(84)	[3]	(33)	[3]
Plant operating expense									895		922		904
Depreciation and amortization expense									609		562		550
Sales, general and other administrative expense									136		140		131
Other operating expenses									81		78		77
Gain (Loss) on Disposition of Assets									0		(222)		0
(Income) from unconsolidated investments in power plants									(30)		(28)		(21)
Income from operations	151	597	122	4	295	705	(193)	195	874		1,002		800
Interest expense, net of interest income									690		725		751
Loss on interest rate derivatives									0		14		145
Debt extinguishment costs and other (income) expense, net									164		45		115
Income (loss) before income taxes and discontinued operations									20		218		(211)
Commodity Margin Riverside Energy Center											73		70
Commodity Margin Broad River Energy Center											52		51
Lease levelization									6		1		12
Contract amortization									14		14		8
Number of significant customers									two		one
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer									1,937		1,668		2,372
Intersegment revenue									5		10		12
Operating revenues									1,942		1,678		2,384
Commodity Margin									1,020		994	[1],[2]	1,061	[1],[2]
Add: Mark-to-market commodity activity, net and other									(50)	[3]	(93)	[3]	113	[3]
Plant operating expense									365		368		380
Depreciation and amortization expense									243		203		192
Sales, general and other administrative expense									37		36		43
Other operating expenses									45		42		41
Gain (Loss) on Disposition of Assets											0
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									280		252		518
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer									2,347		1,857		2,306
Intersegment revenue									(4)		61		23
Operating revenues									2,343		1,918		2,329
Commodity Margin									632		570	[1],[2]	469	[1],[2]
Add: Mark-to-market commodity activity, net and other									51	[3]	87	[3]	(102)	[3]
Plant operating expense									269		247		235
Depreciation and amortization expense									165		142		135
Sales, general and other administrative expense									56		47		43
Other operating expenses									3		5		3
Gain (Loss) on Disposition of Assets											0
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									190		216		(49)
North [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer									1,356		1,280		1,336
Intersegment revenue									33		14		7
Operating revenues									1,389		1,294		1,343
Commodity Margin									712		729	[1],[2]	704	[1],[2]
Add: Mark-to-market commodity activity, net and other									5	[3]	(14)	[3]	(13)	[3]
Plant operating expense									172		206		177
Depreciation and amortization expense									130		134		138
Sales, general and other administrative expense									21		28		24
Other operating expenses									29		29		30
Gain (Loss) on Disposition of Assets											(7)
(Income) from unconsolidated investments in power plants									(30)		(28)		(21)
Income from operations									395		353		343
Southeast [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer									661		673		786
Intersegment revenue									189		80		135
Operating revenues									850		753		921
Commodity Margin									204		245	[1],[2]	240	[1],[2]
Add: Mark-to-market commodity activity, net and other									22	[3]	(33)	[3]	1	[3]
Plant operating expense									120		131		141
Depreciation and amortization expense									73		85		90
Sales, general and other administrative expense									21		29		22
Other operating expenses									4		5		5
Gain (Loss) on Disposition of Assets											(215)
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									8		177		(17)
Consolidation and Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer									0		0		0
Intersegment revenue									(223)		(165)		(177)
Operating revenues									(223)		(165)		(177)
Commodity Margin									0		0	[1],[2]	0	[1],[2]
Add: Mark-to-market commodity activity, net and other									(31)	[3]	(31)	[3]	(32)	[3]
Plant operating expense									(31)		(30)		(29)
Depreciation and amortization expense									(2)		(2)		(5)
Sales, general and other administrative expense									1		0		(1)
Other operating expenses									0		(3)		(2)
Gain (Loss) on Disposition of Assets											0
(Income) from unconsolidated investments in power plants									0		0		0
Income from operations									1		4		5
PJM Settlement, Inc. [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues									820		713		742
Major customer receivables									26		37
Pacific Gas & Electric Company [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenues									694
Major customer receivables									$ 83

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

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

[3] Includes $6 million, $1 million and $12 million of lease levelization and $14 million, $14 million and $8 million of amortization expense for the years ended December 31, 2013, 2012 and 2011, respectively.