Condensed Consolidating Financial Information (Statements of Comprehensive Income) (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Net Income/(Loss)	$ 97	$ 182	$ (80)	$ (67)	$ (290)	$ 138	$ 131	$ (331)	$ 132		$ (352)		$ 315
Unrealized loss on derivatives, net									(45)		8		(163)
Foreign currency translation adjustments, net									(8)		(24)		(1)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax															(11)
Reclassification adjustment for translation gain realized upon sale of Schkopau, net of income tax									0		0		(11)
Available-for-sale securities, net									(7)		3		3
Defined benefit plan, net									(129)		168		(52)
Other comprehensive income/(loss)									(189)		155		(224)
Comprehensive income/(loss)									(57)		(197)		91		91
Less: Comprehensive income/(loss) attributable to noncontrolling interest									8		34		20		20
Comprehensive income/(loss) attributable to NRG Energy, Inc.									(65)		(231)		71
Dividends for preferred shares									56		9		9
Comprehensive (loss)/income, Net of Tax, Available for Common Stockholders									(121)		(240)		62
Guarantor Subsidiaries
Net Income/(Loss)									924		121		535
Unrealized loss on derivatives, net									(49)		(71)		(160)
Foreign currency translation adjustments, net									0		0		0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax													0
Available-for-sale securities, net									0		0		0
Defined benefit plan, net									(5)		(75)		38
Other comprehensive income/(loss)									(44)		4		(198)
Comprehensive income/(loss)									880		125				337
Less: Comprehensive income/(loss) attributable to noncontrolling interest									0		0		0
Comprehensive income/(loss) attributable to NRG Energy, Inc.									880		125		337
Dividends for preferred shares									0		0		0
Comprehensive (loss)/income, Net of Tax, Available for Common Stockholders									880		125		337
Non-Guarantor Subsidiaries
Net Income/(Loss)									426		45		120
Unrealized loss on derivatives, net									(89)		50		(30)
Foreign currency translation adjustments, net									(12)		(20)		(2)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax													(11)
Available-for-sale securities, net									1		0		0
Defined benefit plan, net									104		(63)		0
Other comprehensive income/(loss)									(204)		93		(43)
Comprehensive income/(loss)									222		138				77
Less: Comprehensive income/(loss) attributable to noncontrolling interest									67		27		20
Comprehensive income/(loss) attributable to NRG Energy, Inc.									155		111		57
Dividends for preferred shares									0		0		0
Comprehensive (loss)/income, Net of Tax, Available for Common Stockholders									155		111		57
NRG Energy, Inc.
Net Income/(Loss)									149		(372)		295
Unrealized loss on derivatives, net									(215)		120		(214)
Foreign currency translation adjustments, net									4		(4)		1
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax													0
Available-for-sale securities, net									(8)		3		3
Defined benefit plan, net									30		(30)		14
Other comprehensive income/(loss)									(249)		149		(224)
Comprehensive income/(loss)									(100)		(223)				71
Less: Comprehensive income/(loss) attributable to noncontrolling interest									15		13		0
Comprehensive income/(loss) attributable to NRG Energy, Inc.									(115)		(236)		71
Dividends for preferred shares									56		9		9
Comprehensive (loss)/income, Net of Tax, Available for Common Stockholders									(171)		(245)		62
Eliminations
Net Income/(Loss)									(1,367)	[1]	(146)	[1]	(635)	[1]
Unrealized loss on derivatives, net									308	[1]	(91)	[1]	241	[1]
Foreign currency translation adjustments, net									0	[1]	0	[1]	0	[1]
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax													0	[1]
Available-for-sale securities, net									0	[1]	0	[1]	0	[1]
Defined benefit plan, net									0	[1]	0	[1]	0	[1]
Other comprehensive income/(loss)									308	[1]	(91)	[1]	241	[1]
Comprehensive income/(loss)									(1,059)	[1]	(237)	[1]			(394)	[1]
Less: Comprehensive income/(loss) attributable to noncontrolling interest									(74)	[1]	(6)	[1]	0	[1]
Comprehensive income/(loss) attributable to NRG Energy, Inc.									(985)	[1]	(231)	[1]	(394)	[1]
Dividends for preferred shares									0	[1]	0	[1]	0	[1]
Comprehensive (loss)/income, Net of Tax, Available for Common Stockholders									$ (985)	[1]	$ (231)	[1]	$ (394)	[1]

[1] All significant intercompany transactions have been eliminated in consolidation.