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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, 2023
Dec. 31, 2022
Dec. 31, 2021
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 $ 851 $ 844 $ 1,336
Weighted-Average Assumptions      
Discount rate 4.97% 5.18%  
Interest crediting rate 5.67% 5.21%  
Rate of compensation increase 3.06% 3.06%  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 5.18%   2.55%
Interest crediting rate 5.21% 3.07% 3.13%
Expected return on plan assets 5.55% 4.99% 5.62%
Rate of compensation increase 3.06% 3.06% 3.06%
Pension Benefits | Minimum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 2.89%    
Pension Benefits | Weighted Average      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 4.71%    
Pension Benefits | Maximum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 5.41%    
Pension Benefits | Common/collective trust investment — U.S. equity      
Target allocations      
Target allocation of pension plan assets (as percent) 19.00%    
Pension Benefits | Common/collective trust investment — non-U.S. equity      
Target allocations      
Target allocation of pension plan assets (as percent) 12.00%    
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) 52.00%    
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 19 $ 22  
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 19 22  
Pension Benefits | Significant Observable Inputs (Level 2)      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 483 491  
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 156 155  
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 58 65  
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 81 90  
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 188 181  
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 502 513  
Pension Benefits | Total | Common/collective trust investment — U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 156 155  
Pension Benefits | Total | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 58 65  
Pension Benefits | Total | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 81 90  
Pension Benefits | Total | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 188 181  
Pension Benefits | Total | Short-term investment fund      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 19 22  
Pension Benefits | Net Asset Value      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 851 844  
Pension Benefits | Net Asset Value | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 32 33  
Pension Benefits | Net Asset Value | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 47 55  
Pension Benefits | Net Asset Value | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 243 220  
Pension Benefits | Net Asset Value | Partnerships/joint ventures      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 27 23  
Other Postretirement Benefits Plan      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 0 $ 0 $ 0
Weighted-Average Assumptions      
Discount rate 4.96% 5.19%  
Interest crediting rate 4.66% 4.00%  
Rate of compensation increase 0.00% 0.00%  
Health care trend rate 7.70% 7.00%  
Health care trend rate 4.50% 4.40%  
Year Health care cost trend rate reaches ultimate trend rate 2033 2031  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 5.19% 2.82% 2.81%
Interest crediting rate 4.00% 1.94% 1.62%
Expected return on plan assets 0.00% 0.00% 0.00%
Rate of compensation increase 0.00% 0.00% 0.00%
Health care trend rate 7.70% 7.00%  
Year Health care cost trend rate reaches ultimate trend rate 2033 2031  
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit      
Weighted-Average Assumptions      
Health care trend rate 7.20% 6.90% 7.00%
Health care trend rate 4.50% 4.40% 4.40%
Year Health care cost trend rate reaches ultimate trend rate 2028 2028 2028
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 7.20% 6.90% 7.00%
Year Health care cost trend rate reaches ultimate trend rate 2028 2028 2028