Condensed Consolidating Financial Information - CONSOLIDATING STATEMENTS OF OPERATIONS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating Revenues
Total operating revenues	$ 2,996	$ 2,960	$ 7,626	$ 7,486
Operating Costs and Expenses
Cost of operations	2,153	2,238	5,649	5,512
Depreciation and amortization	91	99	261	331
Impairment losses	0	0	1	74
Selling, general and administrative	210	211	615	587
Reorganization costs	1	27	16	70
Development costs	1	1	5	9
Total operating costs and expenses	2,456	2,576	6,547	6,583
Gain on sale of assets	0	14	2	30
Operating Income/(Loss)	540	398	1,081	933
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Equity in earnings of unconsolidated affiliates	29	20	8	26
Impairment losses on investments	107	1	107	16
Other income, net	17	19	49	12
Loss on debt extinguishment, net	0	(19)	(47)	(22)
Interest expense	(99)	(122)	(318)	(361)
Total other income/(expense)	(160)	(103)	(415)	(361)
Income from Continuing Operations Before Income Taxes	380	295	666	572
Income Tax Expense (Benefit)	6	8	9	19
Income from Continuing Operations	374	287	657	553
Income/(loss) from discontinued operations, net of income tax	(2)	(336)	399	(272)
Net Income/(Loss)	372	(49)	1,056	281
Less: Net income attributable to redeemable noncontrolling interests	0	23	1	1
Net income/(loss) attributable to NRG Energy, Inc.	372	(72)	1,055	280
Eliminations
Operating Revenues
Total operating revenues	(2)	(2)	(8)	(12)
Operating Costs and Expenses
Cost of operations	(2)	(2)	(8)	(13)
Depreciation and amortization	0	0	0	0
Impairment losses			0	0
Selling, general and administrative	0	(73)	0	(73)
Reorganization costs	0	0	0	0
Development costs	0	0	0	(1)
Total operating costs and expenses	(2)	(75)	(8)	(87)
Gain on sale of assets		0	0	0
Operating Income/(Loss)	0	73	0	75
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	(557)	(476)	(1,298)	(1,170)
Equity in earnings of unconsolidated affiliates	0	0	0	0
Impairment losses on investments	0	0	0	0
Other income, net	0	0	0	0
Loss on debt extinguishment, net		0	0	0
Interest expense	0	0	0	0
Total other income/(expense)	(557)	(476)	(1,298)	(1,170)
Income from Continuing Operations Before Income Taxes	(557)	(403)	(1,298)	(1,095)
Income Tax Expense (Benefit)	0	0	0	0
Income from Continuing Operations	(557)	(403)	(1,298)	(1,095)
Income/(loss) from discontinued operations, net of income tax	0	0	0	0
Net Income/(Loss)	(557)	(403)	(1,298)	(1,095)
Less: Net income attributable to redeemable noncontrolling interests		73	0	75
Net income/(loss) attributable to NRG Energy, Inc.	(557)	(476)	(1,298)	(1,170)
Guarantor Subsidiaries
Operating Revenues
Total operating revenues	2,473	2,521	6,382	6,437
Operating Costs and Expenses
Cost of operations	1,728	1,937	4,676	4,794
Depreciation and amortization	52	54	157	177
Impairment losses			1	0
Selling, general and administrative	132	123	366	335
Reorganization costs	0	0	0	3
Development costs	0	0	0	0
Total operating costs and expenses	1,912	2,114	5,200	5,309
Gain on sale of assets		0	1	3
Operating Income/(Loss)	561	407	1,183	1,131
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	20	6	32	14
Equity in earnings of unconsolidated affiliates	0	0	0	0
Impairment losses on investments	0	0	0	0
Other income, net	11	4	19	12
Loss on debt extinguishment, net		0	0	0
Interest expense	(4)	(4)	(11)	(11)
Total other income/(expense)	27	6	40	15
Income from Continuing Operations Before Income Taxes	588	413	1,223	1,146
Income Tax Expense (Benefit)	0	122	0	343
Income from Continuing Operations	588	291	1,223	803
Income/(loss) from discontinued operations, net of income tax	0	16	9	46
Net Income/(Loss)	588	307	1,232	849
Less: Net income attributable to redeemable noncontrolling interests		0	0	0
Net income/(loss) attributable to NRG Energy, Inc.	588	307	1,232	849
Non-Guarantor Subsidiaries
Operating Revenues
Total operating revenues	525	441	1,252	1,061
Operating Costs and Expenses
Cost of operations	421	294	956	713
Depreciation and amortization	32	36	81	128
Impairment losses			0	74
Selling, general and administrative	28	19	56	46
Reorganization costs	0	0	0	0
Development costs	0	(2)	1	0
Total operating costs and expenses	481	347	1,094	961
Gain on sale of assets		14	1	27
Operating Income/(Loss)	44	108	159	127
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Equity in earnings of unconsolidated affiliates	29	21	8	28
Impairment losses on investments	101	1	101	16
Other income, net	1	9	10	(11)
Loss on debt extinguishment, net		0	0	0
Interest expense	(3)	(8)	(12)	(42)
Total other income/(expense)	(74)	21	(95)	(41)
Income from Continuing Operations Before Income Taxes	(30)	129	64	86
Income Tax Expense (Benefit)	1	42	2	26
Income from Continuing Operations	(31)	87	62	60
Income/(loss) from discontinued operations, net of income tax	0	20	5	80
Net Income/(Loss)	(31)	107	67	140
Less: Net income attributable to redeemable noncontrolling interests		(62)	1	(181)
Net income/(loss) attributable to NRG Energy, Inc.	(31)	169	66	321
NRG Energy, Inc. (Note Issuer)
Operating Revenues
Total operating revenues	0	0	0	0
Operating Costs and Expenses
Cost of operations	6	9	25	18
Depreciation and amortization	7	9	23	26
Impairment losses			0	0
Selling, general and administrative	50	142	193	279
Reorganization costs	1	27	16	67
Development costs	1	3	4	10
Total operating costs and expenses	65	190	261	400
Gain on sale of assets		0	0	0
Operating Income/(Loss)	(65)	(190)	(261)	(400)
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	537	470	1,266	1,156
Equity in earnings of unconsolidated affiliates	0	(1)	0	(2)
Impairment losses on investments	6	0	6	0
Other income, net	5	6	20	11
Loss on debt extinguishment, net		(19)	(47)	(22)
Interest expense	(92)	(110)	(295)	(308)
Total other income/(expense)	444	346	938	835
Income from Continuing Operations Before Income Taxes	379	156	677	435
Income Tax Expense (Benefit)	5	(156)	7	(350)
Income from Continuing Operations	374	312	670	785
Income/(loss) from discontinued operations, net of income tax	(2)	(372)	385	(398)
Net Income/(Loss)	372	(60)	1,055	387
Less: Net income attributable to redeemable noncontrolling interests		12	0	107
Net income/(loss) attributable to NRG Energy, Inc.	$ 372	$ (72)	$ 1,055	$ 280