Segment and Significant Customer Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	$ 1,241		$ 1,236
Intersegment revenues	0		0
Total operating revenues	1,241		1,236
Commodity Margin	461		517	[1],[2]
Add: Mark-to-market commodity activity, net and other	(41)	[3]	84	[3]
Plant operating expense	227		221
Depreciation and amortization expense	146		140
Sales, general and other administrative expense	33		33
Other operating expenses	18		21
(Income) loss from unconsolidated investments in power plants	(8)		(9)
Income (loss) from operations	4		195
Interest expense, net of interest income	174		182
Loss on interest rate derivatives	0		14
Debt Extinguishment Costs and Other (Income) Expense, Net	5		14
Income (loss) before income taxes	(175)		(15)
Lease levelization	(16)		(8)
Commodity Margin Riverside Energy Center			8
Commodity Margin Broad River Energy Center			11
Contract amortization	4		4
West [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	383		416
Intersegment revenues	1		4
Total operating revenues	384		420
Commodity Margin	202		208	[1],[2]
Add: Mark-to-market commodity activity, net and other	(37)	[3]	36	[3]
Plant operating expense	93		81
Depreciation and amortization expense	51		50
Sales, general and other administrative expense	4		8
Other operating expenses	9		11
(Income) loss from unconsolidated investments in power plants	0		0
Income (loss) from operations	8		94
Texas [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	407		356
Intersegment revenues	7		20
Total operating revenues	414		376
Commodity Margin	76		109	[1],[2]
Add: Mark-to-market commodity activity, net and other	(11)	[3]	34	[3]
Plant operating expense	68		68
Depreciation and amortization expense	43		35
Sales, general and other administrative expense	17		11
Other operating expenses	1		2
(Income) loss from unconsolidated investments in power plants	0		0
Income (loss) from operations	(64)		27
North [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	306		298
Intersegment revenues	7		3
Total operating revenues	313		301
Commodity Margin	142		144	[1],[2]
Add: Mark-to-market commodity activity, net and other	7	[3]	12	[3]
Plant operating expense	44		45
Depreciation and amortization expense	33		33
Sales, general and other administrative expense	6		6
Other operating expenses	7		9
(Income) loss from unconsolidated investments in power plants	(8)		(9)
Income (loss) from operations	67		72
Southeast [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	145		166
Intersegment revenues	33		22
Total operating revenues	178		188
Commodity Margin	41		56	[1],[2]
Add: Mark-to-market commodity activity, net and other	7	[3]	10	[3]
Plant operating expense	30		33
Depreciation and amortization expense	19		23
Sales, general and other administrative expense	5		8
Other operating expenses	2		1
(Income) loss from unconsolidated investments in power plants	0		0
Income (loss) from operations	(8)		1
Consolidation and Elimination [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues from external customers	0		0
Intersegment revenues	(48)		(49)
Total operating revenues	(48)		(49)
Commodity Margin	0		0	[1],[2]
Add: Mark-to-market commodity activity, net and other	(7)	[3]	(8)	[3]
Plant operating expense	(8)		(6)
Depreciation and amortization expense	0		(1)
Sales, general and other administrative expense	1		0
Other operating expenses	(1)		(2)
(Income) loss from unconsolidated investments in power plants	0		0
Income (loss) from operations	$ 1		$ 1

[1] Our North segment includes Commodity Margin of $8 million for the three months ended March 31, 2012 related to Riverside Energy Center, LLC, which was sold in December 2012.

[2] Our Southeast segment includes Commodity Margin of $11 million for the three months ended March 31, 2012 related to Broad River, which was sold in December 2012.

[3] Includes $(16) million and $(8) million of lease levelization and $4 million and $4 million of amortization expense for the three months ended March 31, 2013 and 2012, respectively.