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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Benefits      
Amounts recognized in balance sheet      
Other current liabilities $ 0 $ 0  
Other non-current liabilities 247 241  
Amounts recognized in accumulated OCI      
Net loss/(gain) 138 90  
Prior service cost/(credit) 2 3  
Total/Net accumulated OCI 140 93  
Other changes in plan assets and benefit obligations recognized in other comprehensive income      
Net actuarial loss/(gain) 50 47  
Amortization of net actuarial (gain)/loss (3) 0 $ (4)
Curtailment 0 (27)  
Prior service credit 0 17  
Amortization of prior service cost 0 0  
Total recognized in OCI 47 37  
Net periodic benefit cost (credit) 0 12 15
Net recognized in net periodic pension cost/(credit) and OCI 47 49  
Expected unrecognized gain that will be amortized from accumulated OCI to net periodic cost over the next fiscal year 5    
Significant components of NRG's domestic pension plan      
Projected benefit obligation 1,397 1,222 1,329
Accumulated benefit obligation 1,362 1,188  
Fair value of plan assets 1,150 981 1,104
Other Postretirement Benefits Plan      
Amounts recognized in balance sheet      
Other current liabilities 7 7  
Other non-current liabilities 86 76  
Amounts recognized in accumulated OCI      
Net loss/(gain) 7 (9)  
Prior service cost/(credit) (43) (53)  
Total/Net accumulated OCI (36) (62)  
Other changes in plan assets and benefit obligations recognized in other comprehensive income      
Net actuarial loss/(gain) 16 (5)  
Amortization of net actuarial (gain)/loss 0 0 1
Curtailment 0 2  
Prior service credit (2) (28)  
Amortization of prior service cost 12 10  
Total recognized in OCI 26 (21)  
Net periodic benefit cost (credit) (9) (15) (5)
Net recognized in net periodic pension cost/(credit) and OCI 17 (36)  
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   14  
Significant components of NRG's domestic pension plan      
Projected benefit obligation 93 83 128
Fair value of plan assets $ 0 $ 0 $ 0