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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, 2021
Dec. 31, 2020
Dec. 31, 2019
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    
Postretirement Benefit Obligation      
Weighted-Average Assumptions      
Year Health care cost trend rate reaches ultimate trend rate 2028 2028  
Significant assumptions used to calculate NRG's benefit expense      
Year Health care cost trend rate reaches ultimate trend rate 2028 2028  
Postretirement Benefit Obligation | Before age 65      
Weighted-Average Assumptions      
Health care trend rate 6.80% 7.20% 7.80%
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 6.80% 7.20% 7.80%
Postretirement Benefit Obligation | Age 65 and after      
Weighted-Average Assumptions      
Health care trend rate 4.40% 4.50%  
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 4.40% 4.50%  
Net Period Benefit Cost/Credit      
Weighted-Average Assumptions      
Year Health care cost trend rate reaches ultimate trend rate 2028 2028 2025
Significant assumptions used to calculate NRG's benefit expense      
Year Health care cost trend rate reaches ultimate trend rate 2028 2028 2025
Net Period Benefit Cost/Credit | Before age 65      
Weighted-Average Assumptions      
Health care trend rate 7.00% 7.50%  
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 7.00% 7.50%  
Net Period Benefit Cost/Credit | Age 65 and after      
Weighted-Average Assumptions      
Health care trend rate 4.40% 4.50% 4.50%
Significant assumptions used to calculate NRG's benefit expense      
Health care trend rate 4.40% 4.50% 4.50%
Pension Benefits      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 1,336 $ 1,272 $ 1,150
Weighted-Average Assumptions      
Discount rate 2.89% 2.56%  
Interest crediting rate 3.07% 3.12%  
Rate of compensation increase 3.06% 3.00%  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 2.55% 3.26%  
Interest crediting rate 3.13% 3.66%  
Expected return on plan assets 5.62% 5.93% 6.35%
Rate of compensation increase 3.06% 3.00% 3.00%
Pension Benefits | Minimum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate     4.38%
Pension Benefits | Maximum      
Significant assumptions used to calculate NRG's benefit expense      
Discount rate     4.20%
Pension Benefits | Common/collective trust investment — U.S. equity      
Target allocations      
Target allocation of pension plan assets (as percent) 17.00%    
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) 15.00%    
Pension Benefits | Common/collective trust investment — fixed income      
Target allocations      
Target allocation of pension plan assets (as percent) 55.00%    
Pension Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 13 $ 13  
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 13  
Pension Benefits | Significant Observable Inputs (Level 2)      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 740 806  
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 221 284  
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 69 113  
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 110 151  
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 340 258  
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 753 819  
Pension Benefits | Total | Common/collective trust investment — U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 221 284  
Pension Benefits | Total | Common/collective trust investment — non-U.S. equity      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 69 113  
Pension Benefits | Total | Common/collective trust investment — non-core assets      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 110 151  
Pension Benefits | Total | Common/collective trust investment — fixed income      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 340 258  
Pension Benefits | Total | Short-term investment fund      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 13 13  
Pension Benefits | Fair Value Measured at Net Asset Value Per Share      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets 1,336 1,272  
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 78 45  
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 65 84  
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 405 289  
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 35  
Other Postretirement Benefits Plan      
Benefit Plans and Other Postretirement Benefits      
Fair value of plan assets $ 0 $ 0 $ 0
Weighted-Average Assumptions      
Discount rate 2.89% 2.54%  
Interest crediting rate 1.94% 1.62%  
Rate of compensation increase 0.00% 0.00%  
Significant assumptions used to calculate NRG's benefit expense      
Discount rate 2.81% 3.26% 4.37%
Interest crediting rate 1.62% 2.28% 0.00%
Expected return on plan assets 0.00% 0.00% 0.00%
Rate of compensation increase 0.00% 0.00% 0.00%