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Benefit Plans and Other Postretirement Benefits - Expected Future Benefit Payments For the Next Five Years and Other (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
STP Defined Benefit Plans |  |  |  |
Curtailment gain | $ 8 |  |  |
Increase to pension liability | $ 32 |  |  |
Company's contributions to 401(k) plans |  |  |  |
Company contributions to defined contribution plans |  | $ 22 | $ 22 | $ 28
South Texas Project |  |  |  |
STP Defined Benefit Plans |  |  |  |
Amount reimbursed to STPNOC towards defined benefit plans |  | 8 | 24 |
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014 |  | 18 |  |
Pension Benefit Payments |  |  |  |
NRG's expected future benefit payments |  |  |  |
2021 |  | 90 |  |
2022 |  | 89 |  |
2023 |  | 87 |  |
2024 |  | 85 |  |
2025 |  | 83 |  |
2026-2030 |  | 386 |  |
STP Defined Benefit Plans |  |  |  |
Curtailment gain |  | 0 | 0 | 7
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |  |
Funded status — STPNOC benefit plans |  | (217) | (247) |
Net periodic benefit credit |  | (8) | 0 | 12
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss) |  | (11) | 47 |
Pension Benefit Payments | South Texas Project |  |  |  |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |  |
Funded status — STPNOC benefit plans |  | (99) | (77) |
Net periodic benefit credit |  | 7 | 9 |
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss) |  | 22 | (13) |
Other Postretirement Benefits Plan |  |  |  |
NRG's expected future benefit payments |  |  |  |
2021 |  | 6 |  |
2022 |  | 6 |  |
2023 |  | 5 |  |
2024 |  | 5 |  |
2025 |  | 5 |  |
2026-2030 |  | 19 |  |
Medicare prescription drug reimbursements |  |  |  |
2021 |  | 0 |  |
2022 |  | 0 |  |
2023 |  | 0 |  |
2024 |  | 0 |  |
2025 |  | 0 |  |
2026-2030 |  | 2 |  |
STP Defined Benefit Plans |  |  |  |
Curtailment gain |  | 0 | 0 | 10
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |  |
Funded status — STPNOC benefit plans |  | (90) | (93) |
Net periodic benefit credit |  | (10) | (9) | $ (15)
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss) |  | 13 | 26 |
Other Postretirement Benefits Plan | South Texas Project |  |  |  |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |  |
Funded status — STPNOC benefit plans |  | (20) | (20) |
Net periodic benefit credit |  | (4) | (4) |
Other changes in plan assets and benefit obligations recognized in other comprehensive income/(loss) |  | $ 5 | $ 6 |
South Texas Project |  |  |  |
STP Defined Benefit Plans |  |  |  |
Ownership interest in STP (as a percent) |  | 44.00% |  |
Percentage of contribution to the retirement plan obligation reimbursed |  | 44.00% |  |

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