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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Annual periodic pension cost          
Curtailment gain $ 0 $ 0 $ (21)    
GenOn          
Fair value of plan assets for pension and other post retirement benefit          
Add: Retained obligation in bankruptcy proceeding       $ (25)  
Pension liability retained       92 $ 120
Pension Benefits          
Annual periodic pension cost          
Service cost benefits earned 26 30 32    
Interest cost on benefit obligation 43 43 53    
Expected return on plan assets (58) (60) (62)    
Amortization of unrecognized net loss 4 2 2    
Net periodic benefit cost 15 15 25    
Pension and other post retirement benefit obligations          
Benefit obligation at January 1 1,241 1,196      
Service cost 26 30 32    
Interest cost 43 43 53    
Plan amendments 0 0      
Actuarial loss/(gain) 77 40      
Employee and retiree contributions 0 0      
Benefit payments (58) (68)      
Benefit obligation at December 31 1,329 1,241 1,196    
Fair value of plan assets for pension and other post retirement benefit          
Fair value of plan assets at January 1 953 916      
Actual return on plan assets 173 72      
Employee and retiree contributions 0 0      
Employer contributions 36 33      
Benefit payments (58) (68)      
Fair value of plan assets at December 31 1,104 953 916    
Funded status at December 31 — excess of obligation over assets       (225) (288)
Less: GenOn postretirement obligation 1,241 1,196 1,196 1,329 1,241
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       (225) (288)
Other Postretirement Benefit          
Annual periodic pension cost          
Service cost benefits earned 1 2 3    
Interest cost on benefit obligation 4 6 9    
Amortization of unrecognized prior service credit (9) (5) (5)    
Amortization of unrecognized net loss (1) 0 1    
Curtailment gain 0 0 (14)    
Net periodic benefit cost (5) 3 (6)    
Pension and other post retirement benefit obligations          
Benefit obligation at January 1 128 178      
Service cost 1 2 3    
Interest cost 4 6 9    
Plan amendments (1) (42)      
Actuarial loss/(gain) 6 (2)      
Employee and retiree contributions 3 3      
Benefit payments (13) (17)      
Benefit obligation at December 31 128 128 178    
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 10 14      
Benefit payments (13) (17)      
Fair value of plan assets at December 31 0 0 0    
Funded status at December 31 — excess of obligation over assets       (128) (128)
Less: GenOn postretirement obligation 128 178 $ 178 128 128
Other Postretirement Benefit | GenOn          
Pension and other post retirement benefit obligations          
Benefit obligation at January 1 46        
Benefit obligation at December 31 38 46      
Fair value of plan assets for pension and other post retirement benefit          
Less: GenOn postretirement obligation $ 46 $ 46   38 46
Add: Retained obligation in bankruptcy proceeding       (25) (25)
Pension liability retained       13 21
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       $ (115) $ (107)