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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, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Benefits      
Amounts recognized in balance sheet      
Other current liabilities $ 0 $ 0  
Other non-current liabilities 217 247  
Amounts recognized in accumulated OCI      
Net loss 127 138  
Prior service cost/(credit) 2 2  
Total/Net accumulated OCI 129 140  
Other changes in plan assets and benefit obligations recognized in other comprehensive income      
Net actuarial (gain)/loss (6) 50  
Amortization of net actuarial (gain) (5) (3) $ 0
Prior service credit 0 0  
Amortization of prior service cost 0 0  
Total recognized in OCI (11) 47  
Net periodic benefit credit (8) 0 12
Net recognized in net periodic pension credit and OCI (19) 47  
Significant components of NRG's domestic pension plan      
Projected benefit obligation 1,489 1,397 1,222
Accumulated benefit obligation 1,455 1,362  
Fair value of plan assets 1,272 1,150 981
Other Postretirement Benefits Plan      
Amounts recognized in balance sheet      
Other current liabilities 5 7  
Other non-current liabilities 85 86  
Amounts recognized in accumulated OCI      
Net loss 6 7  
Prior service cost/(credit) (29) (43)  
Total/Net accumulated OCI (23) (36)  
Other changes in plan assets and benefit obligations recognized in other comprehensive income      
Net actuarial (gain)/loss 0 16  
Amortization of net actuarial (gain) (1) 0 0
Prior service credit 0 (2)  
Amortization of prior service cost 14 12  
Total recognized in OCI 13 26  
Net periodic benefit credit (10) (9) (15)
Net recognized in net periodic pension credit and OCI 3 17  
Significant components of NRG's domestic pension plan      
Projected benefit obligation 90 93 83
Fair value of plan assets $ 0 $ 0 $ 0