Segment Reporting (Details) (USD $) In Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2011		Jun. 30, 2011	Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010		Dec. 31, 2010
Segment Reporting Information
Operating revenues	$ 2,674			$ 2,685		$ 6,947		$ 7,033
Depreciation and amortization	238			210		665		620
Equity in (losses)/earnings of unconsolidated affiliates	16			16		26		41
(Loss)/Income Before Income Taxes	(135)			312		(509)		762
Net loss attributable to noncontrolling interest	0			0		0		(1)
Net Income/(Loss) attributable to NRG Energy, Inc.	(55)			223		306		492
Total Assets	24,760					24,760				26,896
Impairment of Long-Lived Assets Held-for-use	160			0		160		0
Impairment charge on investment	3		11	0		495		0
Loss on debt extinguishment	(32)			(1)		(175)		(2)
Income Tax Reconciliation, Tax Settlements, Domestic						(633)
Reliant Energy [Member] | Wholesale Power Generation | Texas
Segment Reporting Information
Inter-segment sales	597			547		1,230		1,187
Reliant Energy
Segment Reporting Information
Operating revenues	1,637			1,562		3,906		4,020
Depreciation and amortization	24			32		72		91
Equity in (losses)/earnings of unconsolidated affiliates	0			0		0		0
(Loss)/Income Before Income Taxes	65			(20)		368		69
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	65			(20)		368		69
Total Assets	1,623					1,623
Green Mountain Energy [Member] | Wholesale Power Generation | Texas
Segment Reporting Information
Inter-segment sales	110					176
Wholesale Power Generation | Texas
Segment Reporting Information
Operating revenues	822	[1],[2]		1,040	[3]	2,175	[1],[4]	2,602	[5]
Depreciation and amortization	124	[1],[2]		124	[3]	368	[1],[4]	365	[5]
Equity in (losses)/earnings of unconsolidated affiliates	5	[1],[2]		8	[3]	(1)	[1],[4]	19	[5]
(Loss)/Income Before Income Taxes	(56)	[1],[2]		439	[3]	154	[1],[4]	971	[5]
Net loss attributable to noncontrolling interest								(1)	[5]
Net Income/(Loss) attributable to NRG Energy, Inc.	(56)	[1],[2]		439	[3]	154	[1],[4]	972	[5]
Total Assets	12,834	[1],[2]				12,834	[1],[2]
Impairment of Long-Lived Assets Held-for-use						160
Wholesale Power Generation | Northeast
Segment Reporting Information
Operating revenues	299			353		771		837
Depreciation and amortization	33			29		89		92
Equity in (losses)/earnings of unconsolidated affiliates	4			0		9		(1)
(Loss)/Income Before Income Taxes	10			23		(3)		73
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	10			23		(3)		73
Total Assets	1,919					1,919
Wholesale Power Generation | South Central
Segment Reporting Information
Operating revenues	279			166		656		461
Depreciation and amortization	23			17		65		49
Equity in (losses)/earnings of unconsolidated affiliates	0			0		0		0
(Loss)/Income Before Income Taxes	23			8		49		8
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	23			8		49		8
Total Assets	1,279					1,279
Wholesale Power Generation | West
Segment Reporting Information
Operating revenues	49			43		129		110
Depreciation and amortization	3			2		9		8
Equity in (losses)/earnings of unconsolidated affiliates	5			4		10		5
(Loss)/Income Before Income Taxes	24			20		49		34
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	24			20		49		34
Total Assets	2,389					2,389
Wholesale Power Generation | International
Segment Reporting Information
Operating revenues	35			30		108		95
Depreciation and amortization	0			0		0		0
Equity in (losses)/earnings of unconsolidated affiliates	3			4		9		19
(Loss)/Income Before Income Taxes	8			10		26		51
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	6			7		20		36
Total Assets	676					676
Thermal
Segment Reporting Information
Operating revenues	37			40		109		103
Depreciation and amortization	4			3		11		8
Equity in (losses)/earnings of unconsolidated affiliates	0			0		0		0
(Loss)/Income Before Income Taxes	3			3		6		5
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	3			3		6		5
Total Assets	347					347
Corporate
Segment Reporting Information
Operating revenues	227	[6],[7]		(1)		510	[8],[9]	(3)
Depreciation and amortization	27	[6],[7]		3		51	[8],[9]	7
Equity in (losses)/earnings of unconsolidated affiliates	(1)	[6],[7]		0		(1)	[8],[9]	(1)
(Loss)/Income Before Income Taxes	(212)	[6],[7]		(171)		(1,158)	[8],[9]	(449)
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	(130)	[6],[7]		(257)		(337)	[8],[9]	(705)
Total Assets	17,774	[6],[7]				17,774	[6],[7]
Impairment charge on investment	3					495
Elimination
Segment Reporting Information
Operating revenues	(711)			(548)		(1,417)		(1,192)
Depreciation and amortization	0			0		0		0
Equity in (losses)/earnings of unconsolidated affiliates	0			0		0		0
(Loss)/Income Before Income Taxes	0			0		0		0
Net loss attributable to noncontrolling interest								0
Net Income/(Loss) attributable to NRG Energy, Inc.	0			0		0		0
Total Assets	$ (14,081)					$ (14,081)

[1] Includes impairment charge on emission allowances of $160 million.

[2] Includes inter-segment sales of $597 million to Reliant Energy and $110 million to Green Mountain Energy.

[3] Includes inter-segment sales of $547 million to Reliant Energy.

[4] Includes inter-segment sales of $1,230 million to Reliant Energy and $176 million to Green Mountain Energy.

[5] Includes inter-segment sales of $1,187 million to Reliant Energy.

[6] Includes Green Mountain Energy results, and Energy Plus assets as of the September 30, 2011, date of acquisition.

[7] Includes impairment charges on investment of $3 million and loss on debt extinguishment of $32 million.

[8] Includes Green Mountain Energy results.

[9] Includes impairment charges 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.