Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information
Total operating revenues	$ (2,465)	$ (2,461)	$ (4,630)	$ (4,526)
Depreciation and amortization	85	112	170	232
Impairment losses	1	74	1	74
Reorganization costs	2	23	15	43
Gain on sale of assets	1		2	16
Equity in earnings (losses) of unconsolidated affiliates	0	5	(21)	6
Loss on debt extinguishment, net	(47)	(1)	(47)	(3)
(Loss)/income from continuing operations before income taxes	188	32	286	276
(Loss)/income from continuing operations	189	27	283	265
Income/(Loss) from discontinued operations, net of income tax	13	69	401	64
Net (loss)/income	202	96	684	329
Net (loss)/income attributable to NRG Energy, Inc.	201	72	683	351
Eliminations
Segment Reporting Information
Total operating revenues	650	520	915	206
Depreciation and amortization	0	(1)	0	(2)
Impairment losses	0		0	0
Reorganization costs	0	0	0	0
Gain on sale of assets	0		0	0
Equity in earnings (losses) of unconsolidated affiliates		(2)	0	(2)
Loss on debt extinguishment, net	0		0
(Loss)/income from continuing operations before income taxes	1	1	(1)	(1)
(Loss)/income from continuing operations	1	1	(1)	(1)
Income/(Loss) from discontinued operations, net of income tax	0	0	0	0
Net (loss)/income	1	1	(1)	(1)
Net (loss)/income attributable to NRG Energy, Inc.	1	3	(1)	1
Retail | Operating Segments
Segment Reporting Information
Total operating revenues	(1,748)	(1,814)	(3,355)	(3,294)
Depreciation and amortization	32	30	63	56
Impairment losses	1		1	0
Reorganization costs	2	1	3	5
Gain on sale of assets	0		0	0
Equity in earnings (losses) of unconsolidated affiliates		0	0	0
Loss on debt extinguishment, net	0		0
(Loss)/income from continuing operations before income taxes	(280)	(84)	(169)	860
(Loss)/income from continuing operations	(280)	(84)	(170)	860
Income/(Loss) from discontinued operations, net of income tax	0	0	0	0
Net (loss)/income	(280)	(84)	(170)	860
Net (loss)/income attributable to NRG Energy, Inc.	(281)	(84)	(171)	859
Retail | Eliminations
Segment Reporting Information
Total operating revenues	2	4	5	6
Generation | Operating Segments
Segment Reporting Information
Total operating revenues	(1,367)	(1,167)	(2,190)	(1,438)
Depreciation and amortization	45	74	91	160
Impairment losses	0		0	74
Reorganization costs	0	3	1	6
Gain on sale of assets	0		1	2
Equity in earnings (losses) of unconsolidated affiliates		5	(21)	7
Loss on debt extinguishment, net	0		0
(Loss)/income from continuing operations before income taxes	618	252	731	(319)
(Loss)/income from continuing operations	618	252	731	(319)
Income/(Loss) from discontinued operations, net of income tax	0	0	0	0
Net (loss)/income	618	252	731	(319)
Net (loss)/income attributable to NRG Energy, Inc.	618	250	731	(313)
Generation | Eliminations
Segment Reporting Information
Total operating revenues	627	514	862	205
Corporate | Operating Segments
Segment Reporting Information
Total operating revenues	0	0	0	0
Depreciation and amortization	8	9	16	18
Impairment losses	0		0	0
Reorganization costs	0	19	11	32
Gain on sale of assets	1		1	14
Equity in earnings (losses) of unconsolidated affiliates		2	0	1
Loss on debt extinguishment, net	(47)		(47)
(Loss)/income from continuing operations before income taxes	(151)	(137)	(275)	(264)
(Loss)/income from continuing operations	(150)	(142)	(277)	(275)
Income/(Loss) from discontinued operations, net of income tax	13	69	401	64
Net (loss)/income	(137)	(73)	124	(211)
Net (loss)/income attributable to NRG Energy, Inc.	(137)	(97)	124	(196)
Corporate | Eliminations
Segment Reporting Information
Total operating revenues	$ 21	$ 2	$ 48	$ (5)