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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

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