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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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 | $ 1,150 | $ 981 | $ 1,104
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Discount rate | 3.26% | 4.38% |
Rate of compensation increase | 3.00% | 3.00% |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Discount rate |  |  | 4.26%
Expected return on plan assets | 6.35% | 6.17% | 6.85%
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 | $ 12 | $ 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 | 12 | 12 |
Pension Benefits | Significant Observable Inputs (Level 2) |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 721 | 609 |
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 | 233 | 183 |
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 | 73 | 53 |
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 | 143 | 117 |
Pension Benefits | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 272 | 256 |
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 | 733 | 621 |
Pension Benefits | Total | Common/collective trust investment — U.S. equity |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 233 | 183 |
Pension Benefits | Total | Common/collective trust investment — non-U.S. equity |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 73 | 53 |
Pension Benefits | Total | Common/collective trust investment — non-core assets |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 143 | 117 |
Pension Benefits | Total | Common/collective trust investment — fixed income |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 272 | 256 |
Pension Benefits | Total | Short-term investment fund |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 12 | 12 |
Pension Benefits | Fair Value Measured at Net Asset Value Per Share |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 1,150 | 981 |
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 | 84 | 70 |
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 | 24 | 16 |
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 | 279 | 249 |
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 | 30 | 25 |
Other Postretirement Benefits Plan |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | $ 0 | $ 0 | $ 0
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Discount rate | 3.26% | 4.37% |
Rate of compensation increase | 0.00% | 0.00% |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Discount rate |  |  | 4.29%
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 | 4.37% | 3.71% |
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 |  |  |
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Year Health care cost trend rate reaches ultimate trend rate | 2028 | 2025 |
Other Postretirement Benefits Plan | Postretirement Benefit Obligation | Before age 65 |  |  |
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Health care trend rate | 7.50% | 7.80% | 7.00%
Other Postretirement Benefits Plan | Postretirement Benefit Obligation | Age 65 and after |  |  |
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Health care trend rate | 4.50% | 4.50% |
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit |  |  |
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Year Health care cost trend rate reaches ultimate trend rate | 2025 | 2025 | 2025
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit | Before age 65 |  |  |
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Health care trend rate | 7.80% | 8.20% |
Other Postretirement Benefits Plan | Net Period Benefit Cost/Credit | Age 65 and after |  |  |
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Health care trend rate | 4.50% | 4.50% | 5.00%

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