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Benefit Plans and Other Postretirement Benefits - Net Benefit Costs and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 14, 2018	Dec. 31, 2017	Jun. 14, 2017
Annual periodic pension cost
Curtailment gain			$ 80
GenOn
Fair value of plan assets for pension and other post retirement benefit
Add: Retained obligation in bankruptcy proceeding				$ (25)
Pension liability retained				92
Pension Benefits
Annual periodic pension cost
Service cost benefits earned	$ 23	$ 26	30
Interest cost on benefit obligation	44	43	43
Expected return on plan assets	(62)	(58)	(60)
Amortization of unrecognized net loss	0	4	2
Curtailment gain	7	0	0
Net recognized in net periodic pension cost/(credit) and OCI	49	(11)
Net periodic benefit cost			15
Pension and other post retirement benefit obligations
Benefit obligation at January 1	1,329	1,241
Service cost	23	26	30
Interest cost	44	43	43
Plan amendments	17	0
Actuarial (gain)/loss	(95)	77
Employee and retiree contributions	0	0
Curtailment gain	(20)	0
Benefit payments	(76)	(58)
Benefit obligation at December 31	1,222	1,329	1,241
Fair value of plan assets for pension and other post retirement benefit
Fair value of plan assets at January 1	1,104	953
Actual return on plan assets	(80)	173
Employee and retiree contributions	0	0
Employer contributions	33	36
Benefit payments	(76)	(58)
Fair value of plan assets at December 31	981	1,104	953
Funded status at December 31 — excess of obligation over assets				(241)		$ (225)
Less: GenOn postretirement obligation	1,329	1,241	1,241	1,222		1,329
Pension Benefits \| GenOn
Pension and other post retirement benefit obligations
Benefit obligation at January 1	0
Benefit obligation at December 31	0	0
Fair value of plan assets for pension and other post retirement benefit
Less: GenOn postretirement obligation	0	0		0		0
Add: Retained obligation in bankruptcy proceeding				0		0
Pension Benefits \| NRG
Fair value of plan assets for pension and other post retirement benefit
Funded status at December 31 — excess of obligation over assets				(241)		(225)
Other Postretirement Benefit
Annual periodic pension cost
Service cost benefits earned	1	1	2
Interest cost on benefit obligation	4	4	6
Amortization of unrecognized prior service credit	(10)	(9)	(5)
Amortization of unrecognized net loss	0	(1)	0
Curtailment gain	(10)	0	0
Net recognized in net periodic pension cost/(credit) and OCI	(36)	13
Net periodic benefit cost			3
Pension and other post retirement benefit obligations
Benefit obligation at January 1	128	128
Service cost	1	1	2
Interest cost	4	4	6
Plan amendments	(28)	(1)
Actuarial (gain)/loss	(6)	6
Employee and retiree contributions	3	3
Curtailment gain	(7)	0
Benefit payments	(12)	(13)
Benefit obligation at December 31	83	128	128
Fair value of plan assets for pension and other post retirement benefit
Fair value of plan assets at January 1	0	0
Actual return on plan assets	0	0
Employee and retiree contributions	3	3
Employer contributions	9	10
Benefit payments	(12)	(13)
Fair value of plan assets at December 31	0	0	0
Funded status at December 31 — excess of obligation over assets				(83)		(128)
Less: GenOn postretirement obligation	128	128	$ 128	83		128
Other Postretirement Benefit \| GenOn
Pension and other post retirement benefit obligations
Benefit obligation at January 1	38
Benefit obligation at December 31	0	38
Fair value of plan assets for pension and other post retirement benefit
Less: GenOn postretirement obligation	$ 38	$ 38		0	$ 23	38
Add: Retained obligation in bankruptcy proceeding				0		(25)	$ (25)
Other Postretirement Benefit \| NRG
Fair value of plan assets for pension and other post retirement benefit
Funded status at December 31 — excess of obligation over assets				$ (83)		$ (115)