Segment and Significant Customer Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer	$ 1,965		$ 1,241
Intersegment revenue	0		0
Operating revenues	1,965		1,241
Commodity Margin	645		461
Add: Mark-to-market commodity activity, net and other	(37)	[1]	(41)	[1]
Plant operating expense	265		227
Depreciation and amortization expense	153		146
Sales, general and other administrative expense	33		33
Other operating expenses	22		18
(Income) loss from unconsolidated investments in power plants	(9)		(8)
Income from operations	144		4
Interest expense, net of interest income	165		174
Other (income) expense, net	11		5
Loss before income taxes	(32)		(175)
Lease levelization	(29)		(16)
Contract amortization	4		4
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer	491		383
Intersegment revenue	2		1
Operating revenues	493		384
Commodity Margin	202		202
Add: Mark-to-market commodity activity, net and other	29	[1]	(37)	[1]
Plant operating expense	105		95
Depreciation and amortization expense	60		53
Sales, general and other administrative expense	10		7
Other operating expenses	12		9
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	44		1
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer	647		407
Intersegment revenue	12		7
Operating revenues	659		414
Commodity Margin	121		76
Add: Mark-to-market commodity activity, net and other	(46)	[1]	(11)	[1]
Plant operating expense	90		65
Depreciation and amortization expense	42		42
Sales, general and other administrative expense	12		14
Other operating expenses	2		1
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	(71)		(57)
North [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer	639		306
Intersegment revenue	15		7
Operating revenues	654		313
Commodity Margin	267		142
Add: Mark-to-market commodity activity, net and other	(17)	[1]	7	[1]
Plant operating expense	52		43
Depreciation and amortization expense	33		33
Sales, general and other administrative expense	6		6
Other operating expenses	6		7
(Income) loss from unconsolidated investments in power plants	(9)		(8)
Income from operations	162		68
Southeast [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer	188		145
Intersegment revenue	81		33
Operating revenues	269		178
Commodity Margin	55		41
Add: Mark-to-market commodity activity, net and other	6	[1]	7	[1]
Plant operating expense	27		31
Depreciation and amortization expense	18		19
Sales, general and other administrative expense	6		6
Other operating expenses	1		1
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	9		(9)
Intersegment Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue from External Customer	0		0
Intersegment revenue	(110)		(48)
Operating revenues	(110)		(48)
Commodity Margin	0		0
Add: Mark-to-market commodity activity, net and other	(9)	[1]	(7)	[1]
Plant operating expense	(9)		(7)
Depreciation and amortization expense	0		(1)
Sales, general and other administrative expense	(1)		0
Other operating expenses	1		0
(Income) loss from unconsolidated investments in power plants	0		0
Income from operations	$ 0		$ 1

[1] Includes $(29) million and $(16) million of lease levelization and $4 million and $4 million of amortization expense for the three months ended March 31, 2014 and 2013, respectively.