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Benefit Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets and OCI (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 14, 2018
Jun. 14, 2017
Pension Benefits          
Amounts recognized in balance sheet          
Current liabilities $ 0 $ 0      
Non-current liabilities 241 225      
Amounts recognized in accumulated OCI          
Net loss/(gain) 90 53      
Prior service cost/(credit) 3 3      
Total accumulated OCI 93 56      
Other changes in plan assets and benefit obligations recognized in other comprehensive income          
Net actuarial loss/(gain) 47 (37)      
Amortization of net actuarial (gain)/loss 0 (4) $ (2)    
Curtailment (27) 0      
Prior service credit 17 0      
Amortization of prior service cost 0 0      
Total recognized in other comprehensive loss 37 (41)      
Net recognized in net periodic pension cost/(credit) and OCI 49 (11)      
Estimated unrecognized loss for NRG's pension plan that will be amortized from accumulated OCI to net periodic cost over the next fiscal year (less than) 4        
Significant components of NRG's domestic pension plan          
Projected benefit obligation 1,222 1,329 1,241    
Accumulated benefit obligation 1,188 1,255      
Fair value of plan assets 981 1,104 953    
Other Postretirement Benefit          
Amounts recognized in balance sheet          
Current liabilities 7 7      
Non-current liabilities 76 121      
Amounts recognized in accumulated OCI          
Net loss/(gain) (9) (4)      
Prior service cost/(credit) (53) (37)      
Total accumulated OCI (62) (41)      
Other changes in plan assets and benefit obligations recognized in other comprehensive income          
Net actuarial loss/(gain) (5) 6      
Amortization of net actuarial (gain)/loss 0 1 0    
Curtailment 2 0      
Prior service credit (28) (1)      
Amortization of prior service cost 10 9      
Total recognized in other comprehensive loss (21) 15      
Net recognized in net periodic pension cost/(credit) and OCI (36) 13      
Expected unrecognized gain that will be amortized from accumulated OCI to net periodic cost over the next fiscal year 1        
Expected amortization of unrecognized prior service credit over the next fiscal Year   13      
Significant components of NRG's domestic pension plan          
Projected benefit obligation 83 128 128    
Fair value of plan assets 0 0 $ 0    
GenOn          
Amounts recognized in balance sheet          
Pension benefit obligations 92        
Liability retained other post-employment and retiree health and welfare benefits 25        
Amounts recognized in accumulated OCI          
Net loss/(gain) (28)        
GenOn | Pension Benefits          
Amounts recognized in balance sheet          
Current liabilities 0 0      
Non-current liabilities 0 0      
Liability retained other post-employment and retiree health and welfare benefits 0 0      
Significant components of NRG's domestic pension plan          
Projected benefit obligation 0 0      
GenOn | Other Postretirement Benefit          
Amounts recognized in balance sheet          
Current liabilities 0 3      
Non-current liabilities 0 10      
Liability retained other post-employment and retiree health and welfare benefits 0 25     $ 25
Significant components of NRG's domestic pension plan          
Projected benefit obligation 0 38   $ 23  
NRG | Pension Benefits          
Amounts recognized in balance sheet          
Current liabilities 0 0      
Non-current liabilities 241 225      
NRG | Other Postretirement Benefit          
Amounts recognized in balance sheet          
Current liabilities 7 4      
Non-current liabilities 76 111      
Discontinued Operations | Pension Benefits          
Amounts recognized in accumulated OCI          
Total accumulated OCI 0 (22)      
Other changes in plan assets and benefit obligations recognized in other comprehensive income          
Total recognized in other comprehensive loss 0 15      
Net recognized in net periodic pension cost/(credit) and OCI 0 0      
Discontinued Operations | Other Postretirement Benefit          
Amounts recognized in accumulated OCI          
Total accumulated OCI 0 10      
Other changes in plan assets and benefit obligations recognized in other comprehensive income          
Total recognized in other comprehensive loss 0 2      
Net recognized in net periodic pension cost/(credit) and OCI 0 1      
Continuing Operations | Pension Benefits          
Amounts recognized in accumulated OCI          
Total accumulated OCI 93 34      
Other changes in plan assets and benefit obligations recognized in other comprehensive income          
Total recognized in other comprehensive loss 37 (26)      
Net recognized in net periodic pension cost/(credit) and OCI 12 15      
Continuing Operations | Other Postretirement Benefit          
Amounts recognized in accumulated OCI          
Total accumulated OCI (62) (31)      
Other changes in plan assets and benefit obligations recognized in other comprehensive income          
Total recognized in other comprehensive loss (21) 17      
Net recognized in net periodic pension cost/(credit) and OCI $ (15) $ (5)