Condensed Consolidating Financial Information - CONSOLIDATING STATEMENTS OF OPERATIONS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Operating Revenues
Total operating revenues	$ 2,465	$ 2,461	$ 4,630	$ 4,526
Operating Costs and Expenses
Cost of operations	1,845	1,889	3,496	3,274
Depreciation and amortization	85	112	170	232
Impairment losses	1	74	1	74
Selling, general and administrative	211	200	405	376
Reorganization costs	2	23	15	43
Development costs	2	3	4	8
Total operating costs and expenses	2,146	2,301	4,091	4,007
Gain on sale of assets	1	14	2	16
Operating Income/(Loss)	320	174	541	535
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Equity in earnings/(losses) of unconsolidated affiliates	0	5	(21)	6
Other income/(expense), net	20	(23)	32	(23)
Loss on debt extinguishment, net	(47)	(1)	(47)	(3)
Interest expense	(105)	(123)	(219)	(239)
Total other income/(expense)	(132)	(142)	(255)	(259)
Income from Continuing Operations Before Income Taxes	188	32	286	276
Income tax expense/(benefit)	(1)	5	3	11
Income from Continuing Operations	189	27	283	265
Income/(loss) from discontinued operations	13	69	401	64
Net (loss)/income	202	96	684	329
Less: Net income attributable to noncontrolling interest and redeemable interests	1	24	1	(22)
Net income attributable to NRG Energy, Inc.	201	72	683	351
Eliminations
Operating Revenues
Total operating revenues	(7)	(2)	(6)	(10)
Operating Costs and Expenses
Cost of operations	(7)	(3)	(6)	(11)
Depreciation and amortization	0	0	0	0
Impairment losses	0	0	0	0
Selling, general and administrative	0	0	0	0
Reorganization costs	0	0	0	0
Development costs	0	(1)	0	(1)
Total operating costs and expenses	(7)	(4)	(6)	(12)
Gain on sale of assets	0	0	0	0
Operating Income/(Loss)	0	2	0	2
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	(432)	(360)	(741)	(693)
Equity in earnings/(losses) of unconsolidated affiliates		0	0	0
Other income/(expense), net	0	0	0	0
Loss on debt extinguishment, net	0	0	0	0
Interest expense	0	0	0	0
Total other income/(expense)	(432)	(360)	(741)	(693)
Income from Continuing Operations Before Income Taxes	(432)	(358)	(741)	(691)
Income tax expense/(benefit)	0	0	0	0
Income from Continuing Operations	(432)	(358)	(741)	(691)
Income/(loss) from discontinued operations	0	0	0	0
Net (loss)/income	(432)	(358)	(741)	(691)
Less: Net income attributable to noncontrolling interest and redeemable interests	0	2	0	2
Net income attributable to NRG Energy, Inc.	(432)	(360)	(741)	(693)
Guarantor Subsidiaries
Operating Revenues
Total operating revenues	2,140	2,172	3,909	3,916
Operating Costs and Expenses
Cost of operations	1,590	1,702	2,948	2,857
Depreciation and amortization	51	63	105	123
Impairment losses	1	0	1	0
Selling, general and administrative	112	108	234	212
Reorganization costs	0	1	0	3
Development costs	0	0	0	0
Total operating costs and expenses	1,754	1,874	3,288	3,195
Gain on sale of assets	0	0	1	3
Operating Income/(Loss)	386	298	622	724
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	2	7	12	8
Equity in earnings/(losses) of unconsolidated affiliates		0	0	0
Other income/(expense), net	4	3	8	8
Loss on debt extinguishment, net	0	0	0	0
Interest expense	(3)	(4)	(7)	(7)
Total other income/(expense)	3	6	13	9
Income from Continuing Operations Before Income Taxes	389	304	635	733
Income tax expense/(benefit)	0	108	0	221
Income from Continuing Operations	389	196	635	512
Income/(loss) from discontinued operations	0	15	9	30
Net (loss)/income	389	211	644	542
Less: Net income attributable to noncontrolling interest and redeemable interests	0	0	0	0
Net income attributable to NRG Energy, Inc.	389	211	644	542
Non-Guarantor Subsidiaries
Operating Revenues
Total operating revenues	332	291	727	620
Operating Costs and Expenses
Cost of operations	252	195	535	419
Depreciation and amortization	26	40	49	92
Impairment losses	0	74	0	74
Selling, general and administrative	12	16	28	27
Reorganization costs	0	0	0	0
Development costs	1	1	1	2
Total operating costs and expenses	291	326	613	614
Gain on sale of assets	1	14	1	13
Operating Income/(Loss)	42	(21)	115	19
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0	0	0	0
Equity in earnings/(losses) of unconsolidated affiliates		5	(21)	7
Other income/(expense), net	8	(29)	9	(36)
Loss on debt extinguishment, net	0	0	0	0
Interest expense	(5)	(13)	(9)	(34)
Total other income/(expense)	3	(37)	(21)	(63)
Income from Continuing Operations Before Income Taxes	45	(58)	94	(44)
Income tax expense/(benefit)	1	(71)	1	(16)
Income from Continuing Operations	44	13	93	(28)
Income/(loss) from discontinued operations	0	80	5	60
Net (loss)/income	44	93	98	32
Less: Net income attributable to noncontrolling interest and redeemable interests	1	(56)	1	(119)
Net income attributable to NRG Energy, Inc.	43	149	97	151
NRG Energy, Inc. (Note Issuer)
Operating Revenues
Total operating revenues	0	0	0	0
Operating Costs and Expenses
Cost of operations	10	(5)	19	9
Depreciation and amortization	8	9	16	17
Impairment losses	0	0	0	0
Selling, general and administrative	87	76	143	137
Reorganization costs	2	22	15	40
Development costs	1	3	3	7
Total operating costs and expenses	108	105	196	210
Gain on sale of assets	0	0	0	0
Operating Income/(Loss)	(108)	(105)	(196)	(210)
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	430	353	729	685
Equity in earnings/(losses) of unconsolidated affiliates		0	0	(1)
Other income/(expense), net	8	3	15	5
Loss on debt extinguishment, net	(47)	(1)	(47)	(3)
Interest expense	(97)	(106)	(203)	(198)
Total other income/(expense)	294	249	494	488
Income from Continuing Operations Before Income Taxes	186	144	298	278
Income tax expense/(benefit)	(2)	(32)	2	(194)
Income from Continuing Operations	188	176	296	472
Income/(loss) from discontinued operations	13	(26)	387	(26)
Net (loss)/income	201	150	683	446
Less: Net income attributable to noncontrolling interest and redeemable interests	0	78	0	95
Net income attributable to NRG Energy, Inc.	$ 201	$ 72	$ 683	$ 351