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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Low |  |  |
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 | $ 1,104 | $ 953 | $ 916
Significant assumptions used to calculate NRG's benefit obligations |  |  |
Discount rate | 3.71% | 4.26% |
Rate of compensation increase | 3.00% | 3.00% |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Discount rate | 4.26% | 4.52% | 4.16%
Expected return on plan assets | 6.85% | 6.65% | 6.36%
Rate of compensation increase | 3.00% | 3.00% | 3.45%
Other Postretirement Benefit |  |  |
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 | 3.71% | 4.29% |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Discount rate | 4.29% | 4.55% | 4.20%
Expected return on plan assets | 0.00% | 0.00% | 0.00%
Rate of compensation increase | 0.00% | 0.00% | 0.00%
Common/collective trust investment — U.S. equity | Pension Benefits |  |  |
Target allocations |  |  |
Target allocation of pension plan assets (as percent) | 22.00% |  |
Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Target allocations |  |  |
Target allocation of pension plan assets (as percent) | 14.00% |  |
Common/collective trust investment — non-core assets | Pension Benefits |  |  |
Target allocations |  |  |
Target allocation of pension plan assets (as percent) | 19.00% |  |
Common/collective trust investment — fixed income | Pension Benefits |  |  |
Target allocations |  |  |
Target allocation of pension plan assets (as percent) | 45.00% |  |
Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Significant Observable Inputs (Level 2) | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Postretirement Benefit Obligation | Other Postretirement Benefit |  |  |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Year Health care cost trend rate reaches ultimate trend rate | 2025 |  | 2025
Postretirement Benefit Obligation | Before age 65 | Other Postretirement Benefit |  |  |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Health care trend rate | 8.20% | 7.00% |
Postretirement Benefit Obligation | Age 65 and after | Other Postretirement Benefit |  |  |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Health care trend rate | 4.50% | 5.00% |
Net Period Benefit Cost/Credit | Other Postretirement Benefit |  |  |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Year Health care cost trend rate reaches ultimate trend rate | 2025 | 2025 | 2023
Net Period Benefit Cost/Credit | Before age 65 | Other Postretirement Benefit |  |  |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Health care trend rate | 7.00% | 7.25% | 8.60%
Net Period Benefit Cost/Credit | Age 65 and after | Other Postretirement Benefit |  |  |
Significant assumptions used to calculate NRG's benefit expense |  |  |
Health care trend rate | 5.00% | 5.00% | 5.00%
Other than measured at net asset value practical expedient | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | $ 735 | $ 651 |
Other than measured at net asset value practical expedient | Common/collective trust investment — U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 256 | 283 |
Other than measured at net asset value practical expedient | Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 66 | 71 |
Other than measured at net asset value practical expedient | Common/collective trust investment — non-core assets | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 178 |  |
Other than measured at net asset value practical expedient | Common/collective trust investment — global equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  | 104 |
Other than measured at net asset value practical expedient | Common/collective trust investment — fixed income | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 230 | 190 |
Other than measured at net asset value practical expedient | Short-term investment fund | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 5 | 3 |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 5 | 3 |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 0 | 0 |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 0 | 0 |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-core assets | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 0 |  |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — global equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  | 0 |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — fixed income | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 0 | 0 |
Other than measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investment fund | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 5 | 3 |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 730 | 648 |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 256 | 283 |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 66 | 71 |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-core assets | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 178 |  |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — global equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  | 104 |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 230 | 190 |
Other than measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Short-term investment fund | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 0 | 0 |
Measured at net asset value practical expedient | Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 94 | 78 |
Measured at net asset value practical expedient | Common/collective trust investment — fixed income | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 233 | 193 |
Measured at net asset value practical expedient | Partnerships/joint ventures | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets | 42 | 31 |
Measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common/collective trust investment — fixed income | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Measured at net asset value practical expedient | Quoted Prices in Active Markets for Identical Assets (Level 1) | Partnerships/joint ventures | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — non-U.S. equity | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Common/collective trust investment — fixed income | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |
Measured at net asset value practical expedient | Significant Observable Inputs (Level 2) | Partnerships/joint ventures | Pension Benefits |  |  |
Benefit Plans and Other Postretirement Benefits |  |  |
Fair value of plan assets |  |  |

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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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