Condensed Consolidating Financial Information - CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 202	$ 96	$ 684	$ 329
Other Comprehensive Income/(Loss), net of tax
Unrealized gain on derivatives, net	0	5	0	19
Foreign currency translation adjustments, net	(1)	(4)	0	(6)
Available-for-sale securities, net	1	1	1	1
Defined benefit plans, net	(3)	(1)	(6)	(2)
Other comprehensive (loss)/income	(3)	1	(5)	12
Comprehensive Income	199	97	679	341
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable interests	1	26	1	(12)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	198	71	678	353
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net Income	(432)	(358)	(741)	(691)
Other Comprehensive Income/(Loss), net of tax
Unrealized gain on derivatives, net		(5)		(22)
Foreign currency translation adjustments, net	2	9		14
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0	0	0	0
Other comprehensive (loss)/income	2	4	0	(8)
Comprehensive Income	(430)	(354)	(741)	(699)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable interests	0	2	0	2
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	(430)	(356)	(741)	(701)
Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net Income	389	211	644	542
Other Comprehensive Income/(Loss), net of tax
Unrealized gain on derivatives, net		0		0
Foreign currency translation adjustments, net	(1)	(4)		(6)
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0	0	0	0
Other comprehensive (loss)/income	(1)	(4)	0	(6)
Comprehensive Income	388	207	644	536
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable interests	0	0	0	0
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	388	207	644	536
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net Income	44	93	98	32
Other Comprehensive Income/(Loss), net of tax
Unrealized gain on derivatives, net		4		20
Foreign currency translation adjustments, net	(1)	(4)		(6)
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0	0	0	0
Other comprehensive (loss)/income	(1)	0	0	14
Comprehensive Income	43	93	98	46
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable interests	1	(56)	1	(119)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	42	149	97	165
NRG Energy, Inc. (Note Issuer)
Condensed Financial Statements, Captions [Line Items]
Net Income	201	150	683	446
Other Comprehensive Income/(Loss), net of tax
Unrealized gain on derivatives, net		6		21
Foreign currency translation adjustments, net	(1)	(5)		(8)
Available-for-sale securities, net	1	1	1	1
Defined benefit plans, net	(3)	(1)	(6)	(2)
Other comprehensive (loss)/income	(3)	1	(5)	12
Comprehensive Income	198	151	678	458
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable interests	0	80	0	105
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	$ 198	$ 71	$ 678	$ 353