Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information
Operating revenues	$ 2,996	$ 2,960	$ 7,626	$ 7,486
Depreciation and amortization	91	99	261	331
Impairment losses	0	0	1	74
Reorganization costs	1	27	16	70
(Loss)/gain on sale of assets	0		2	30
Equity in earnings (losses) of unconsolidated affiliates	29	20	8	26
Loss on debt extinguishment, net	0	(19)	(47)	(22)
Income/(loss) from continuing operations before income taxes	380	295	666	572
Income/(loss) from continuing operations	374	287	657	553
Loss from discontinued operations, net of income tax	(2)	(336)	399	(272)
Net Income/(Loss)	372	(49)	1,056	281
Net (loss)/income attributable to NRG Energy, Inc.	372	(72)	1,055	280
Eliminations
Segment Reporting Information
Operating revenues	(522)	(722)	(1,437)	(927)
Depreciation and amortization	0	2	0	0
Impairment losses			0	0
Reorganization costs	0	0	0	0
(Loss)/gain on sale of assets	0		0	0
Equity in earnings (losses) of unconsolidated affiliates	0	(3)	0	(5)
Loss on debt extinguishment, net		0	0
Income/(loss) from continuing operations before income taxes	1	(7)	0	(4)
Income/(loss) from continuing operations	1	(7)	0	(4)
Loss from discontinued operations, net of income tax	0	0	0	0
Net Income/(Loss)	1	(7)	0	(4)
Net (loss)/income attributable to NRG Energy, Inc.	1	(5)	0	0
Retail | Operating Segments
Segment Reporting Information
Operating revenues	2,544	2,203	5,899	5,497
Depreciation and amortization	37	30	100	86
Impairment losses			1	0
Reorganization costs	1	6	4	10
(Loss)/gain on sale of assets	(1)		(1)	0
Equity in earnings (losses) of unconsolidated affiliates	0	0	0	0
Loss on debt extinguishment, net		0	0
Income/(loss) from continuing operations before income taxes	422	(128)	253	732
Income/(loss) from continuing operations	422	(128)	252	732
Loss from discontinued operations, net of income tax	0	0	0	0
Net Income/(Loss)	422	(128)	252	732
Net (loss)/income attributable to NRG Energy, Inc.	422	(128)	251	731
Retail | Eliminations
Segment Reporting Information
Operating revenues	(1)	(1)	(6)	(7)
Generation | Operating Segments
Segment Reporting Information
Operating revenues	974	1,478	3,164	2,916
Depreciation and amortization	47	60	138	220
Impairment losses			0	74
Reorganization costs	0	3	1	10
(Loss)/gain on sale of assets	1		2	1
Equity in earnings (losses) of unconsolidated affiliates	29	20	8	27
Loss on debt extinguishment, net		0	0
Income/(loss) from continuing operations before income taxes	64	575	795	256
Income/(loss) from continuing operations	63	574	794	255
Loss from discontinued operations, net of income tax	0	0	0	0
Net Income/(Loss)	63	574	794	255
Net (loss)/income attributable to NRG Energy, Inc.	63	559	794	246
Generation | Eliminations
Segment Reporting Information
Operating revenues	(513)	(739)	(1,375)	(944)
Corporate | Operating Segments
Segment Reporting Information
Operating revenues	0	1	0	0
Depreciation and amortization	7	7	23	25
Impairment losses			0	0
Reorganization costs	0	18	11	50
(Loss)/gain on sale of assets	0		1	29
Equity in earnings (losses) of unconsolidated affiliates	0	3	0	4
Loss on debt extinguishment, net		(19)	(47)
Income/(loss) from continuing operations before income taxes	(107)	(145)	(382)	(412)
Income/(loss) from continuing operations	(112)	(152)	(389)	(430)
Loss from discontinued operations, net of income tax	(2)	(336)	399	(272)
Net Income/(Loss)	(114)	(488)	10	(702)
Net (loss)/income attributable to NRG Energy, Inc.	(114)	(498)	10	(697)
Corporate | Eliminations
Segment Reporting Information
Operating revenues	$ (8)	$ 18	$ (56)	$ 24