Condensed Consolidating Financial Information - Statements of Operations (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Revenues
Total operating revenues		$ 1,992	$ 2,960	$ 2,461	$ 2,065	$ 2,154	$ 2,618	$ 2,281	$ 2,021	$ 9,478	$ 9,074	$ 8,915
Operating Costs and Expenses
Cost of operations										7,108	6,886	6,676
Depreciation and amortization										421	596	756
Impairment losses										99	1,534	483
Selling, general and administrative										799	836	1,032
Reorganization costs										90	44
Development costs										11	22	48
Total operating costs and expenses										8,528	9,918	8,995
Other income - affiliate	$ 84									0	87	193
Gain on sale of assets										32	16	(80)
Operating Income/(Loss)		49	398	174	361	(1,200)	275	214	(30)	982	(741)	33
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries										0	0	0
Equity in earnings/(losses) of unconsolidated affiliates										9	(14)	(18)
Impairment losses on investments										(15)	(79)	(268)
Other income, net										18	51	47
Loss on debt extinguishment, net										(44)	(49)	(142)
Interest expense										(483)	(557)	(583)
Total other expense										(515)	(648)	(964)
Income/(Loss) from Continuing Operations Before Income Taxes										467	(1,389)	(931)
Income tax (benefit)/expense										7	(44)	25
Net Income/(Loss) from Continuing Operations		(93)	288	27	238	(1,390)	163	60	(178)	460	(1,345)	(956)
(Loss)/income from discontinued operations, net of income tax		80	(336)	69	(5)	(265)	0	(702)	(25)	(192)	(992)	65
Net Income/(Loss)		(13)	(48)	96	233	(1,655)	163	(642)	(203)	268	(2,337)	(891)
Less: Net loss attributable to noncontrolling interests and redeemable noncontrolling interests		$ (2)	$ 24	$ 24	$ (46)	$ (120)	$ (8)	$ (16)	$ (40)	0	(184)	(117)
Net Income/(Loss) Attributable to NRG Energy, Inc.										268	(2,153)	(774)
Eliminations
Operating Revenues
Total operating revenues										(26)	(48)	(74)
Operating Costs and Expenses
Cost of operations										(26)	(46)	(80)
Depreciation and amortization										0	0	0
Impairment losses										0	0	0
Selling, general and administrative										(74)	(2)	0
Reorganization costs										0	0
Development costs										(1)	0	0
Total operating costs and expenses										(101)	(48)	(80)
Other income - affiliate											0	0
Gain on sale of assets										0	0	0
Operating Income/(Loss)										75	0	6
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries										(1,314)	(46)	(324)
Equity in earnings/(losses) of unconsolidated affiliates										0	0	(10)
Impairment losses on investments										0	0	0
Other income, net										0	0	0
Loss on debt extinguishment, net										0	0	0
Interest expense										0	0	0
Total other expense										(1,314)	(46)	(334)
Income/(Loss) from Continuing Operations Before Income Taxes										(1,239)	(46)	(328)
Income tax (benefit)/expense										0	0	0
Net Income/(Loss) from Continuing Operations										(1,239)	(46)	(328)
(Loss)/income from discontinued operations, net of income tax										0	0	0
Net Income/(Loss)										(1,239)	(46)	(328)
Less: Net loss attributable to noncontrolling interests and redeemable noncontrolling interests										75	0	(4)
Net Income/(Loss) Attributable to NRG Energy, Inc.										(1,314)	(46)	(324)
Guarantor Subsidiaries
Operating Revenues
Total operating revenues										8,119	7,818	7,539
Operating Costs and Expenses
Cost of operations										6,147	5,998	5,581
Depreciation and amortization										238	343	500
Impairment losses										6	1,346	370
Selling, general and administrative										462	410	430
Reorganization costs										4	6
Development costs										0	0	0
Total operating costs and expenses										6,857	8,103	6,881
Other income - affiliate											0	0
Gain on sale of assets										4	4	(1)
Operating Income/(Loss)										1,266	(281)	657
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries										23	18	(50)
Equity in earnings/(losses) of unconsolidated affiliates										0	0	5
Impairment losses on investments										0	0	0
Other income, net										32	9	5
Loss on debt extinguishment, net										0	0	0
Interest expense										(14)	(14)	(15)
Total other expense										41	13	(55)
Income/(Loss) from Continuing Operations Before Income Taxes										1,307	(268)	602
Income tax (benefit)/expense										372	(598)	(1)
Net Income/(Loss) from Continuing Operations										935	330	603
(Loss)/income from discontinued operations, net of income tax										62	91	86
Net Income/(Loss)										997	421	689
Less: Net loss attributable to noncontrolling interests and redeemable noncontrolling interests										0	0	0
Net Income/(Loss) Attributable to NRG Energy, Inc.										997	421	689
Non-Guarantor Subsidiaries
Operating Revenues
Total operating revenues										1,385	1,304	1,450
Operating Costs and Expenses
Cost of operations										959	862	1,116
Depreciation and amortization										150	221	230
Impairment losses										93	188	113
Selling, general and administrative										63	64	144
Reorganization costs										0	0
Development costs										1	4	18
Total operating costs and expenses										1,266	1,339	1,621
Other income - affiliate											0	0
Gain on sale of assets										28	12	0
Operating Income/(Loss)										147	(23)	(171)
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries										0	0	0
Equity in earnings/(losses) of unconsolidated affiliates										10	(10)	(9)
Impairment losses on investments										(15)	(75)	(252)
Other income, net										(13)	14	15
Loss on debt extinguishment, net										0	0	(4)
Interest expense										(49)	(91)	(85)
Total other expense										(67)	(162)	(335)
Income/(Loss) from Continuing Operations Before Income Taxes										80	(185)	(506)
Income tax (benefit)/expense										19	(62)	28
Net Income/(Loss) from Continuing Operations										61	(123)	(534)
(Loss)/income from discontinued operations, net of income tax										75	(420)	0
Net Income/(Loss)										136	(543)	(534)
Less: Net loss attributable to noncontrolling interests and redeemable noncontrolling interests										(181)	(168)	(169)
Net Income/(Loss) Attributable to NRG Energy, Inc.										317	(375)	(365)
NRG Energy, Inc. (Note Issuer)
Operating Revenues
Total operating revenues										0	0	0
Operating Costs and Expenses
Cost of operations										28	72	59
Depreciation and amortization										33	32	26
Impairment losses										0	0	0
Selling, general and administrative										348	364	458
Reorganization costs										86	38
Development costs										11	18	30
Total operating costs and expenses										506	524	573
Other income - affiliate											87	193
Gain on sale of assets										0	0	(79)
Operating Income/(Loss)										(506)	(437)	(459)
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries										1,291	28	374
Equity in earnings/(losses) of unconsolidated affiliates										(1)	(4)	(4)
Impairment losses on investments										0	(4)	(16)
Other income, net										(1)	28	27
Loss on debt extinguishment, net										(44)	(49)	(138)
Interest expense										(420)	(452)	(483)
Total other expense										825	(453)	(240)
Income/(Loss) from Continuing Operations Before Income Taxes										319	(890)	(699)
Income tax (benefit)/expense										(384)	616	(2)
Net Income/(Loss) from Continuing Operations										703	(1,506)	(697)
(Loss)/income from discontinued operations, net of income tax										(329)	(663)	(21)
Net Income/(Loss)										374	(2,169)	(718)
Less: Net loss attributable to noncontrolling interests and redeemable noncontrolling interests										106	(16)	56
Net Income/(Loss) Attributable to NRG Energy, Inc.										$ 268	$ (2,153)	$ (774)