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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, 2019 | Dec. 31, 2018 | Dec. 31, 2017
One-percentage-point change in assumed health care cost trend rates |  |  |  |
Effect on postretirement benefit obligation, 1-Percentage-Point Increase |  | $ 7 |  |
Effect on postretirement benefit obligation, 1-Percentage-Point Decrease |  | (5) |  |
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 | $ 28 | $ 56
South Texas Project |  |  |  |
STP Defined Benefit Plans |  |  |  |
Amount reimbursed to STPNOC towards defined benefit plans |  | 24 | 13 |
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014 |  | 7 |  |
Pension Benefit Payments |  |  |  |
NRG's expected future benefit payments |  |  |  |
2020 |  | 84 |  |
2021 |  | 86 |  |
2022 |  | 86 |  |
2023 |  | 86 |  |
2024 |  | 86 |  |
2025-2029 |  | 402 |  |
STP Defined Benefit Plans |  |  |  |
Curtailment gain |  | 0 | (7) | 0
Increase to pension liability |  | 0 | (20) |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |  |
Funded status — STPNOC benefit plans |  | (247) | (241) |
Net periodic benefit cost (credit) |  | 0 | 12 | 15
Total recognized in other comprehensive loss |  | (47) | (37) |
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 |  | (77) | (78) |
Net periodic benefit cost (credit) |  | 9 | 8 |
Total recognized in other comprehensive loss |  | (13) | (7) |
Other Postretirement Benefits Plan |  |  |  |
NRG's expected future benefit payments |  |  |  |
2020 |  | 7 |  |
2021 |  | 6 |  |
2022 |  | 6 |  |
2023 |  | 6 |  |
2024 |  | 6 |  |
2025-2029 |  | 19 |  |
Medicare prescription drug reimbursements |  |  |  |
2020 |  | 0 |  |
2021 |  | 0 |  |
2022 |  | 0 |  |
2023 |  | 0 |  |
2024 |  | 0 |  |
2025-2029 |  | 2 |  |
STP Defined Benefit Plans |  |  |  |
Curtailment gain |  | 0 | 10 | 0
Increase to pension liability |  | 0 | (7) |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |  |
Funded status — STPNOC benefit plans |  | (93) | (83) |
Net periodic benefit cost (credit) |  | (9) | (15) | $ (5)
Total recognized in other comprehensive loss |  | (26) | 21 |
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) | (19) |
Net periodic benefit cost (credit) |  | (4) | (7) |
Total recognized in other comprehensive loss |  | $ 6 | $ 2 |
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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