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Benefit Plans and Other Postretirement Benefits - Fair Values of Pension Assets By Asset Category and Level Within the Fair Value Hierarchy (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Minimum      
Significant assumptions used to calculate NRG's benefit expense      
Aon Hewitt above median yield curve discount rate (in years) 6 months    
Maximum      
Significant assumptions used to calculate NRG's benefit expense      
Aon Hewitt above median yield curve discount rate (in years) 99 years    
Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 1,272 $ 1,150 $ 981
Weighted-Average Assumptions      
Discount rate 2.56% 3.26%  
Interest crediting rate 3.12% 1.62%  
Rate of compensation increase 3.00% 3.00%  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 3.26%    
Interest crediting rate 3.66%    
Expected return on plan assets 5.93% 6.35% 6.17%
Rate of compensation increase 3.00% 3.00% 3.00%
Pension Benefits | Minimum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate   4.38% 3.71%
Pension Benefits | Maximum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate   4.20% 4.04%
Pension Benefits | Common/collective trust investment — U.S. equity      
Target allocations      
Target allocation of pension plan assets (as percent) 20.00%    
Pension Benefits | Common/collective trust investment — non-U.S. equity      
Target allocations      
Target allocation of pension plan assets (as percent) 13.00%    
Pension Benefits | Common/collective trust investment — non-core assets      
Target allocations      
Target allocation of pension plan assets (as percent) 17.00%    
Pension Benefits | Common/collective trust investment — fixed income      
Target allocations      
Target allocation of pension plan assets (as percent) 50.00%    
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 13 $ 12  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investment fund      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 13 12  
Pension Benefits | Significant Observable Inputs (Level 2)      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 806 721  
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 284 233  
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 113 73  
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 151 143  
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 258 272  
Pension Benefits | Significant Observable Inputs (Level 2) | Short-term investment fund      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 0 0  
Pension Benefits | Total      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 819 733  
Pension Benefits | Total | Common/collective trust investment — U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 284 233  
Pension Benefits | Total | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 113 73  
Pension Benefits | Total | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 151 143  
Pension Benefits | Total | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 258 272  
Pension Benefits | Total | Short-term investment fund      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 13 12  
Pension Benefits | Fair Value Measured at Net Asset Value Per Share      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 1,272 1,150  
Pension Benefits | Fair Value Measured at Net Asset Value Per Share | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 45 84  
Pension Benefits | Fair Value Measured at Net Asset Value Per Share | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 84 24  
Pension Benefits | Fair Value Measured at Net Asset Value Per Share | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 289 279  
Pension Benefits | Fair Value Measured at Net Asset Value Per Share | Partnerships/joint ventures      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 35 30  
Other Postretirement Benefits Plan      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 0 $ 0 $ 0
Weighted-Average Assumptions      
Discount rate 2.54% 3.26%  
Rate of compensation increase 0.00% 0.00%  
Significant assumptions used to calculate NRG's benefit expense      
Expected return on plan assets 0.00% 0.00% 0.00%
Rate of compensation increase 0.00% 0.00% 0.00%
Other Postretirement Benefits Plan | Minimum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 3.26% 4.37% 3.71%
Interest crediting rate 2.28%    
Other Postretirement Benefits Plan | Maximum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate     4.08%
Other Postretirement Benefits Plan | Postretirement Benefit Obligation      
Weighted-Average Assumptions      
Year Health care cost trend rate reaches ultimate trend rate 2028 2028  
Other Postretirement Benefits Plan | Postretirement Benefit Obligation | Before age 65      
Weighted-Average Assumptions      
Health care trend rate 7.20% 7.50% 8.20%
Other Postretirement Benefits Plan | Postretirement Benefit Obligation | Age 65 and after      
Weighted-Average Assumptions      
Health care trend rate 4.50% 4.50%  
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit      
Weighted-Average Assumptions      
Year Health care cost trend rate reaches ultimate trend rate 2028 2025 2025
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit | Before age 65      
Weighted-Average Assumptions      
Health care trend rate 7.50% 7.80%  
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit | Age 65 and after      
Weighted-Average Assumptions      
Health care trend rate 4.50% 4.50% 4.50%