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Benefit Plans and Other Postretirement Benefits (Tables) | 12 Months Ended
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Dec. 31, 2020
Retirement Benefits [Abstract] |
Schedule of Net Benefit Cost (Credit) Related to Pension and Other Postretirement Benefit Plans Components | The annual net periodic benefit cost/(credit) related to NRG's pension and other postretirement benefit plans include the following components:
| Year Ended December 31,
| Pension Benefits
(In millions) | 2020 |  | 2019 |  | 2018
Service cost benefits earned | $ | 10 |  |  | $ | 10 |  |  | $ | 23 |
Interest cost on benefit obligation | 38 |  |  | 46 |  |  | 44 |
Expected return on plan assets | (61) |  |  | (59) |  |  | (62) |
Amortization of unrecognized net loss | 5 |  |  | 3 |  |  | — |
Settlement/curtailment expense | — |  |  | — |  |  | 7 |
Net periodic benefit (credit)/cost | $ | (8) |  |  | $ | — |  |  | $ | 12 |
| Year Ended December 31,
| Other Postretirement Benefits
(In millions) | 2020 |  | 2019 |  | 2018
Service cost benefits earned | $ | — |  |  | $ | 1 |  |  | $ | 1 |
Interest cost on benefit obligation | 3 |  |  | 3 |  |  | 4 |
Amortization of unrecognized prior service cost | (14) |  |  | (13) |  |  | (10) |
Amortization of unrecognized net loss | 1 |  |  | — |  |  | — |
Curtailment gain | — |  |  | — |  |  | (10) |
Net periodic benefit credit | $ | (10) |  |  | $ | (9) |  |  | $ | (15) |
Schedule of Comparison of Pension Benefit obligation, Other Postretirement Benefit Obligations and Related Plan Assets on a Combined Basis | A comparison of the pension benefit obligation, other postretirement benefit obligations and related plan assets for NRG's plans on a combined basis is as follows:
| As of December 31,
| Pension Benefits |  | Other Postretirement Benefits
(In millions) | 2020 |  | 2019 |  | 2020 |  | 2019
Benefit obligation at January 1 | $ | 1,397 |  |  | $ | 1,222 |  |  | $ | 93 |  |  | $ | 83 |
Service cost | 10 |  |  | 10 |  |  | — |  |  | 1 |
Interest cost | 38 |  |  | 46 |  |  | 3 |  |  | 3 |
Plan amendments | — |  |  | — |  |  | — |  |  | (2) |
Actuarial loss | 126 |  |  | 207 |  |  | — |  |  | 16 |
Employee and retiree contributions | — |  |  | — |  |  | 3 |  |  | 4 |
Benefit payments | (82) |  |  | (88) |  |  | (9) |  |  | (12) |
Benefit obligation at December 31 | 1,489 |  |  | 1,397 |  |  | 90 |  |  | 93 |
Fair value of plan assets at January 1 | 1,150 |  |  | 981 |  |  | — |  |  | — |
Actual return on plan assets | 193 |  |  | 216 |  |  | — |  |  | — |
Employee and retiree contributions | — |  |  | — |  |  | 3 |  |  | 4 |
Employer contributions | 11 |  |  | 41 |  |  | 6 |  |  | 7 |
Benefit payments | (82) |  |  | (88) |  |  | (9) |  |  | (11) |
Fair value of plan assets at December 31 | 1,272 |  |  | 1,150 |  |  | — |  |  | — |
Funded status at December 31 — excess of obligation over assets | $ | (217) |  |  | $ | (247) |  |  | $ | (90) |  |  | $ | (93) |
Schedule of Amounts Recognized in Balance Sheet | Amounts recognized in NRG's balance sheets were as follows:
| As of December 31,
| Pension Benefits |  | Other Postretirement Benefits
(In millions) | 2020 |  | 2019 |  | 2020 |  | 2019
Other current liabilities | $ | — |  |  | $ | — |  |  | $ | 5 |  |  | $ | 7 |
Other non-current liabilities | 217 |  |  | 247 |  |  | 85 |  |  | 86 |
Schedule of Amounts Recognized in OCI Not Yet Recognized as Components of Net Periodic Benefit Costs | Amounts recognized in NRG's accumulated OCI that have not yet been recognized as components of net periodic benefit cost were as follows:
| As of December 31,
| Pension Benefits |  | Other Postretirement Benefits
(In millions) | 2020 |  | 2019 |  | 2020 |  | 2019
Net loss | $ | 127 |  |  | $ | 138 |  |  | $ | 6 |  |  | $ | 7 |
Prior service cost/(credit) | 2 |  |  | 2 |  |  | (29) |  |  | (43) |
Total accumulated OCI | $ | 129 |  |  | $ | 140 |  |  | $ | (23) |  |  | $ | (36) |
Schedule of Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income | Other changes in plan assets and benefit obligations recognized in OCI were as follows:
| Year Ended December 31,
| Pension Benefits |  | Other Postretirement Benefits
(In millions) | 2020 |  | 2019 |  | 2020 |  | 2019
Net actuarial (gain)/loss | $ | (6) |  |  | $ | 50 |  |  | $ | — |  |  | $ | 16 |
Amortization of net actuarial (gain) | (5) |  |  | (3) |  |  | (1) |  |  | — |
Prior service credit | — |  |  | — |  |  | — |  |  | (2) |
Amortization of prior service cost | — |  |  | — |  |  | 14 |  |  | 12 |
Total recognized in OCI | $ | (11) |  |  | $ | 47 |  |  | $ | 13 |  |  | $ | 26 |
Net periodic benefit credit | (8) |  |  | — |  |  | (10) |  |  | (9) |
Net recognized in net periodic pension credit and OCI | $ | (19) |  |  | $ | 47 |  |  | $ | 3 |  |  | $ | 17 |
Schedule of Benefit Obligations Significant Components | The following table presents the balances of significant components of NRG's pension plan:
| As of December 31,
| Pension Benefits
(In millions) | 2020 |  | 2019
Projected benefit obligation | $ | 1,489 |  |  | $ | 1,397 |
Accumulated benefit obligation | 1,455 |  |  | 1,362 |
Fair value of plan assets | 1,272 |  |  | 1,150 |
Schedule of Fair Value of Pension Plan Assets by Asset Category and Level within the Fair Value Hierarchy | NRG's market-related value of its plan assets is the fair value of the assets. The fair values of the Company's pension plan assets by asset category and their level within the fair value hierarchy are as follows:
| Fair Value Measurements as of December 31, 2020
(In millions) | Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Observable Inputs (Level 2) |  | Total
Common/collective trust investment — U.S. equity | $ | — |  |  | $ | 284 |  |  | $ | 284 |
Common/collective trust investment — non-U.S. equity | — |  |  | 113 |  |  | 113 |
Common/collective trust investment — non-core assets | — |  |  | 151 |  |  | 151 |
Common/collective trust investment — fixed income | — |  |  | 258 |  |  | 258 |
Short-term investment fund | 13 |  |  | — |  |  | 13 |
Subtotal fair value | $ | 13 |  |  | $ | 806 |  |  | $ | 819 |
Measured at net asset value practical expedient: |  |  |  |  |
Common/collective trust investment — non-U.S. equity |  |  |  |  | 45 |
Common/collective trust investment — fixed income |  |  |  |  | 289 |
Common/collective trust investment — non-core assets |  |  |  |  | 84 |
Partnerships/joint ventures |  |  |  |  | 35 |
Total fair value |  |  |  |  | $ | 1,272 |
| Fair Value Measurements as of December 31, 2019
(In millions) | Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Observable Inputs (Level 2) |  | Total
Common/collective trust investment — U.S. equity | $ | — |  |  | $ | 233 |  |  | $ | 233 |
Common/collective trust investment — non-U.S. equity | — |  |  | 73 |  |  | 73 |
Common/collective trust investment — non-core assets | — |  |  | 143 |  |  | 143 |
Common/collective trust investment — fixed income | — |  |  | 272 |  |  | 272 |
Short-term investment fund | 12 |  |  | — |  |  | 12 |
Subtotal fair value | $ | 12 |  |  | $ | 721 |  |  | $ | 733 |
Measured at net asset value practical expedient: |  |  |  |  |
Common/collective trust investment — non-U.S. equity |  |  |  |  | 84 |
Common/collective trust investment — fixed income |  |  |  |  | 279 |
Common/collective trust investment — non-core assets |  |  |  |  | 24 |
Partnerships/joint ventures |  |  |  |  | 30 |
Total fair value |  |  |  |  | $ | 1,150 |
Schedule of Assumptions Used to Calculate Benefit Expense | The following table presents the significant assumptions used to calculate NRG's benefit obligations:
| As of December 31,
| Pension Benefits |  | Other Postretirement Benefits
Weighted-Average Assumptions | 2020 |  | 2019 |  | 2020 |  | 2019
Discount rate | 2.56 | % |  | 3.26 | % |  | 2.54 | % |  | 3.26 | %
Interest crediting rate | 3.12 | % |  | — |  |  | 1.62 | % |  | — |
Rate of compensation increase | 3.00 | % |  | 3.00 | % |  | — | % |  | — | %
Health care trend rate | — |  |  | — |  |  | 7.2% grading to 4.5% in 2028 |  | 7.5% grading to 4.5% in 2028
The following table presents the significant assumptions used to calculate NRG's benefit expense:
| As of December 31,
| Pension Benefits |  | Other Postretirement Benefits
Weighted-Average Assumptions | 2020 |  | 2019 |  | 2018 |  | 2020 |  | 2019 |  | 2018
Discount rate | 3.26 | % |  | 4.38%/4.20% |  | 3.71%/4.04% |  | 3.26% |  | 4.37% |  | 3.71% /4.08%
Interest crediting rate | 3.66 | % |  | — |  |  | — |  |  | 2.28 | % |  | — |  |  | — |
Expected return on plan assets | 5.93 | % |  | 6.35 | % |  | 6.17 | % |  | — |  |  | — |  |  | — |
Rate of compensation increase | 3.00 | % |  | 3.00 | % |  | 3.00 | % |  | — |  |  | — |  |  | — |
Health care trend rate | — |  |  | — |  |  | — |  |  | 7.5% grading to 4.5% in 2028 |  | 7.8% grading to 4.5% in 2025 |  | 8.2% grading to 4.5% in 2025
Schedule of Target Allocation of Pension Plan Assets | The target allocations of NRG's pension plan assets were as follows for the year ended December 31, 2020:
U.S. equity | 20 | %
Non-U.S. equity | 13 | %
Non-core assets | 17 | %
U.S. fixed income | 50 | %
Schedule of Performance Benchmarks | Performance benchmarks are composed of the following indices:
Asset Class |  | Index
U.S. equities |  | Dow Jones U.S. Total Stock Market Index
Non-U.S. equities |  | MSCI All Country World Ex-U.S. IMI Index
Non-core assets(a) |  | Various (per underlying asset class)
Fixed income securities |  | Barclays Short, Intermediate and Long Credits/Barclays Strips 20+ Index
(a)Non-Core Assets are defined as diversifying asset classes approved by the Investment Committee that are intended to enhance returns and/or reduce volatility of the U.S. and non-U.S. equities. Asset classes considered Non-Core include, but may not be limited to: Emerging Market Equity, Emerging Market Debt, Non-US Developed Market Small Cap, High Yield Fixed Income, Real Estate, Bank Loans, Global Infrastructure and other Alternatives.
Schedule of Expected Benefit Payments | NRG's expected future benefit payments for each of the next five years, and in the aggregate for the five years thereafter, are as follows:
|  |  | Other Postretirement Benefit
(In millions) | Pension Benefit Payments |  | Benefit Payments |  | Medicare Prescription Drug Reimbursements
2021 | $ | 90 |  |  | $ | 6 |  |  | $ | — |
2022 | 89 |  |  | 6 |  |  | — |
2023 | 87 |  |  | 5 |  |  | — |
2024 | 85 |  |  | 5 |  |  | — |
2025 | 83 |  |  | 5 |  |  | — |
2026-2030 | 386 |  |  | 19 |  |  | 2 |
Schedule of Benefit Costs and Other Changes Recognized in the Financial Statements Related to its Interest in STP | The Company has recognized the following in its statement of financial position, statement of operations and accumulated OCI related to its 44% interest in STP:
| As of December 31,
| Pension Benefits |  | Other Postretirement Benefits
(In millions) | 2020 |  | 2019 |  | 2020 |  | 2019
Funded status — STPNOC benefit plans | $ | (99) |  |  | $ | (77) |  |  | $ | (20) |  |  | $ | (20) |
Net periodic benefit cost/(credit) | 7 |  |  | 9 |  |  | (4) |  |  | (4) |
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss) | 22 |  |  | (13) |  |  | 5 |  |  | 6 |
Defined Contribution Plan Contributions | The Company's contributions to these plans were as follows:
| Year Ended December 31,
(In millions) | 2020 |  | 2019 |  | 2018
Company contributions to defined contribution plans | $ | 22 |  |  | $ | 22 |  |  | $ | 28 |

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