(Loss) Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Numerator:
Net (loss)/Income attributable to NRG Energy, Inc.	$ (1)	$ (55)	$ 43	$ 306
Preferred stock dividends	(2)	(2)	(7)	(7)
Net (loss)/Income available for common stockholders	(3)	(57)	36	299
Weighted average number of common shares outstanding, basic	228	240	228	243
Net Income (Loss) Available to Common Stockholders, Diluted	$ (3)	$ (57)	$ 36	$ 299
Incremental Common Shares Attributable to Share-based Payment Arrangements	0	0	2	2
Weighted average number of common shares outstanding-diluted	228	240	230	245
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net (loss)/income per weighted average common share - basic	$ (0.01)	$ (0.24)	$ 0.16	$ 1.23
Net (loss)/income per weighted average common share - diluted	$ (0.01)	$ (0.24)	$ 0.16	$ 1.22
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	27	23	22	23
Preferred Stock Instrument, Interest Rate, Stated Percentage	3.625%	3.625%	3.625%	3.625%	3.625%
Non Qualified Stock Options And Performance Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	11	7	6	7
Embedded Derivative Of Redeemable Perpetual Preferred Stock 3625 [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	16	16	16	16