Condensed Consolidating Financial Information - COMPREHENSIVE INCOME/(LOSS) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements, Captions [Line Items]
Net Income	$ 96	$ (642)	$ 329	$ (845)
Other Comprehensive Income, net of tax
Unrealized gain on derivatives, net	5	(5)	19	(1)
Foreign currency translation adjustments, net	(4)	1	(6)	8
Available-for-sale securities, net	1	1	1	1
Defined benefit plans, net	(1)	27	(2)	27
Other comprehensive income	1	24	12	35
Comprehensive Income/(Loss)	97	(618)	341	(810)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	26	(17)	(12)	(56)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	71	(601)	353	(754)
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net Income	(360)	106	(692)	53
Other Comprehensive Income, net of tax
Unrealized gain on derivatives, net	(5)	5	(22)	0
Foreign currency translation adjustments, net	9	0	14	(9)
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0	(29)	0	(29)
Other comprehensive income	4	(24)	(8)	(38)
Comprehensive Income/(Loss)	(356)	82	(700)	15
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	2	(35)	2	(34)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	(358)	117	(702)	49
Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net Income	210	186	543	263
Other Comprehensive Income, net of tax
Unrealized gain on derivatives, net	0	0	0	0
Foreign currency translation adjustments, net	(4)	0	(6)	5
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0	0	0	0
Other comprehensive income	(4)	0	(6)	5
Comprehensive Income/(Loss)	206	186	537	268
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	0	0	0	0
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	206	186	537	268
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net Income	95	(336)	32	(396)
Other Comprehensive Income, net of tax
Unrealized gain on derivatives, net	4	(6)	20	(1)
Foreign currency translation adjustments, net	(4)	1	(6)	5
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0	28	0	29
Other comprehensive income	0	23	14	33
Comprehensive Income/(Loss)	95	(313)	46	(363)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	(57)	(10)	(119)	(47)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	152	(303)	165	(316)
NRG Energy, Inc. (Note Issuer)
Condensed Financial Statements, Captions [Line Items]
Net Income	151	(598)	446	(765)
Other Comprehensive Income, net of tax
Unrealized gain on derivatives, net	6	(4)	21	0
Foreign currency translation adjustments, net	(5)	0	(8)	7
Available-for-sale securities, net	1	1	1	1
Defined benefit plans, net	(1)	28	(2)	27
Other comprehensive income	1	25	12	35
Comprehensive Income/(Loss)	152	(573)	458	(730)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	81	28	105	25
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	$ 71	$ (601)	$ 353	$ (755)

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.

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Amount after tax and reclassification adjustments, of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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