Condensed Consolidating Financial Information - COMPREHENSIVE INCOME/(LOSS) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Net Income/(Loss)	$ (642)	$ (276)	$ (845)	$ (229)
Other Comprehensive Income/(Loss), net of tax
Unrealized (loss)/gain on derivatives, net	(5)	(3)	(1)	(35)
Foreign currency translation adjustments, net	1	(3)	8	3
Available-for-sale securities, net	1	(2)	1	1
Defined benefit plans, net	27	0	27	1
Other comprehensive income/(loss)	24	(8)	35	(30)
Comprehensive Income/(Loss)	(618)	(284)	(810)	(259)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	(17)	(16)	(56)	(68)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	(601)	(268)	(754)	(191)
Dividends for preferred shares	0	0	0	5
Gain on redemption of preferred shares	0	(78)	0	(78)
Comprehensive Income/(Loss) Available for Common Stockholders	(601)	(190)	(754)	(118)
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net Income/(Loss)	53	(28)	91	(244)
Other Comprehensive Income/(Loss), net of tax
Unrealized (loss)/gain on derivatives, net	5	6	0	0
Foreign currency translation adjustments, net	0	5	(9)	(3)
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	(29)		(29)	0
Other comprehensive income/(loss)	(24)	11	(38)	(3)
Comprehensive Income/(Loss)	29	(17)	53	(247)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	(35)	(46)	(34)	(53)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	64	29	87	(194)
Dividends for preferred shares				0
Gain on redemption of preferred shares		0		0
Comprehensive Income/(Loss) Available for Common Stockholders		29		(194)
Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net Income/(Loss)	163	204	207	369
Other Comprehensive Income/(Loss), net of tax
Unrealized (loss)/gain on derivatives, net	0	0	0	0
Foreign currency translation adjustments, net	0	(2)	5	2
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	0		0	1
Other comprehensive income/(loss)	0	(2)	5	3
Comprehensive Income/(Loss)	163	202	212	372
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	0	0	0	0
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	163	202	212	372
Dividends for preferred shares				0
Gain on redemption of preferred shares		0		0
Comprehensive Income/(Loss) Available for Common Stockholders		202		372
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net Income/(Loss)	(260)	(212)	(378)	(201)
Other Comprehensive Income/(Loss), net of tax
Unrealized (loss)/gain on derivatives, net	(6)	(5)	(1)	(55)
Foreign currency translation adjustments, net	1	(2)	5	2
Available-for-sale securities, net	0	0	0	0
Defined benefit plans, net	28		29	0
Other comprehensive income/(loss)	23	(7)	33	(53)
Comprehensive Income/(Loss)	(237)	(219)	(345)	(254)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	(10)	(1)	(47)	(51)
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	(227)	(218)	(298)	(203)
Dividends for preferred shares				0
Gain on redemption of preferred shares		0		0
Comprehensive Income/(Loss) Available for Common Stockholders		(218)		(203)
NRG Energy, Inc. (Note Issuer)
Condensed Financial Statements, Captions [Line Items]
Net Income/(Loss)	(598)	(240)	(765)	(153)
Other Comprehensive Income/(Loss), net of tax
Unrealized (loss)/gain on derivatives, net	(4)	(4)	0	20
Foreign currency translation adjustments, net	0	(4)	7	2
Available-for-sale securities, net	1	(2)	1	1
Defined benefit plans, net	28		27	0
Other comprehensive income/(loss)	25	(10)	35	23
Comprehensive Income/(Loss)	(573)	(250)	(730)	(130)
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	28	31	25	36
Comprehensive Income/(Loss) Attributable to NRG Energy, Inc.	$ (601)	(281)	$ (755)	(166)
Dividends for preferred shares				5
Gain on redemption of preferred shares		(78)		(78)
Comprehensive Income/(Loss) Available for Common Stockholders		$ (203)		$ (93)