Segment Reporting (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating revenues	$ 2,331		$ 2,674		$ 6,359		$ 6,947
Depreciation and amortization	239		238		703		665
Equity in earnings of unconsolidated affiliates	4		16		26		26
Income/(Loss) Before Income Taxes	(105)		(135)		(185)		(509)
Net Income attributable to NRG Energy, Inc.	(1)		(55)		43		306
Total Assets	27,220				27,220				26,715
Impairment charge on emission allowances	0		160		0		160
Impairment charge on investment	(1)		(3)		(2)		(495)
Loss on debt extinguishment	41		32		41		175
Valuation Allowance, Deferred Tax Asset, Change in Amount							633
Retail Site [Member]
Segment Reporting Information [Line Items]
Operating revenues	1,856	[1]	1,861	[1]	4,492	[2]	4,409	[3]
Depreciation and amortization	41		48		126		114
Equity in earnings of unconsolidated affiliates	0		0		0		0
Income/(Loss) Before Income Taxes	(300)		36		504		347
Net Income attributable to NRG Energy, Inc.	(300)		36		504		350
Total Assets	3,179				3,179
Inter-segment sales	3		4		3		4
Conventional Generation [Member] | Texas [Member]
Segment Reporting Information [Line Items]
Operating revenues	877	[1]	817	[1]	1,462	[2]	2,149	[3]
Depreciation and amortization	115		117		343		347
Equity in earnings of unconsolidated affiliates	0		0		0		0
Income/(Loss) Before Income Taxes	299		(45)	[4]	(202)		193	[5]
Net Income attributable to NRG Energy, Inc.	299		(45)	[4]	(202)		193	[5]
Total Assets	12,109				12,109
Inter-segment sales	1,126		697		1,287		1,401
Conventional Generation [Member] | Northeast
Segment Reporting Information [Line Items]
Operating revenues	274	[1]	298	[1]	598	[2]	770	[3]
Depreciation and amortization	32		33		96		89
Equity in earnings of unconsolidated affiliates	3		4		11		9
Income/(Loss) Before Income Taxes	33		6		(20)		(13)
Net Income attributable to NRG Energy, Inc.	33		6		(20)		(13)
Total Assets	1,945				1,945
Inter-segment sales	6		3		51		5
Conventional Generation [Member] | South Central
Segment Reporting Information [Line Items]
Operating revenues	270		279		653		656
Depreciation and amortization	23		23		69		65
Equity in earnings of unconsolidated affiliates	0		0		0		0
Income/(Loss) Before Income Taxes	19		21		0		46
Net Income attributable to NRG Energy, Inc.	19		21		0		46
Total Assets	1,676				1,676
Inter-segment sales	0		0		0		0
Conventional Generation [Member] | West
Segment Reporting Information [Line Items]
Operating revenues	87		45		185		122
Depreciation and amortization	3		2		8		7
Equity in earnings of unconsolidated affiliates	4		4		6		9
Income/(Loss) Before Income Taxes	35		27		42		51
Net Income attributable to NRG Energy, Inc.	35		27		42		51
Total Assets	909				909
Inter-segment sales	0		0		0		0
Conventional Generation [Member] | All Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	68	[1]	81	[6]	262	[2]	244	[3]
Depreciation and amortization	4		4		12		11
Equity in earnings of unconsolidated affiliates	2		3		8		9
Income/(Loss) Before Income Taxes	9		7		29		20
Net Income attributable to NRG Energy, Inc.	9		5		25		14
Total Assets	692				692
Inter-segment sales	12		6		55		15
Alternative Energy [Member]
Segment Reporting Information [Line Items]
Operating revenues	56	[1]	10	[6]	114	[2]	33	[3]
Depreciation and amortization	18		7		41		22
Equity in earnings of unconsolidated affiliates	(5)		6		1		0
Income/(Loss) Before Income Taxes	0		(12)		(22)		(42)
Net Income attributable to NRG Energy, Inc.	(9)		(12)		(40)		(42)
Total Assets	5,615				5,615
Inter-segment sales	10		5		18		13
Corporate
Segment Reporting Information [Line Items]
Operating revenues	4	[1]	5	[6]	11	[2]	9	[3]
Depreciation and amortization	3		4		8		10
Equity in earnings of unconsolidated affiliates	0		(1)		0		(1)
Income/(Loss) Before Income Taxes	(200)	[7]	(175)	[8]	(516)	[9]	(1,111)	[10]
Net Income attributable to NRG Energy, Inc.	(87)	[7]	(93)	[8]	(266)	[9]	(293)	[10]
Total Assets	18,076				18,076
Inter-segment sales	4		5		4		5
Elimination
Segment Reporting Information [Line Items]
Operating revenues	(1,161)		(722)		(1,418)		(1,445)
Depreciation and amortization	0		0		0		0
Equity in earnings of unconsolidated affiliates	0		0		0		0
Income/(Loss) Before Income Taxes	0		0		0		0
Net Income attributable to NRG Energy, Inc.	0		0		0		0
Total Assets	$ (16,981)				$ (16,981)

[1] (a) Includes intersegment sales and derivative gains and losses of:$3 $1,126 $6 $— $— $12 $10 $4

[2] (f) Includes intersegment sales and derivative gains and losses of:$3 $1,287 $51 $— $— $55 $18 $4

[3] (h) Includes intersegment sales and derivative gains and losses of:$4 $1,401 $5 $— $— $15 $13 $5

[4] ncludes impairment charge on emission allowances of $160 million.

[5] ncludes impairment charge on emission allowances of $160 million

[6] (c) Includes intersegment sales and derivative gains and losses of:$4 $697 $3 $— $— $6 $5 $5

[7] Includes loss on debt extinguishment of $41 million.

[8] Includes loss on debt extinguishment of $32 million.

[9] ncludes loss on debt extinguishment of $41 million.

[10] Includes impairment charge on investment of $495 million, loss on debt extinguishment of $175 million, and tax benefit of $633 million resulting from the resolution of the federal tax audit.