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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 | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
One-percentage-point change in assumed health care cost trend rates |  |  |
Effect on postretirement benefit obligation, 1-Percentage-Point Increase | $ 5 |  |
Effect on postretirement benefit obligation, 1-Percentage-Point Decrease | (4) |  |
Company's contributions to 401(k) plans |  |  |
Company contributions to defined contribution plans | $ 28 | $ 56 | $ 55
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% |  |
Amount reimbursed to STPNOC towards defined benefit plans | $ 13 | 8 |
Expected reimbursement of contribution to retirement plan obligations to STPNOC in 2014 | 18 |  |
Pension Benefit Payments |  |  |
NRG's expected future benefit payments |  |  |
2019 | 72 |  |
2020 | 76 |  |
2021 | 79 |  |
2022 | 82 |  |
2023 | 85 |  |
2024-2028 | 418 |  |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |
Funded status — STPNOC benefit plans | (241) | (225) |
Net periodic benefit costs |  |  | 15
Total recognized in other comprehensive loss | (37) | 41 |
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 | (78) | (76) |
Net periodic benefit costs | 8 | 8 |
Total recognized in other comprehensive loss | (7) | (6) |
Other Postretirement Benefit |  |  |
NRG's expected future benefit payments |  |  |
2019 | 7 |  |
2020 | 7 |  |
2021 | 7 |  |
2022 | 6 |  |
2023 | 6 |  |
2024-2028 | 26 |  |
Medicare prescription drug reimbursements |  |  |
2019 | 0 |  |
2020 | 0 |  |
2021 | 0 |  |
2022 | 0 |  |
2023 | 0 |  |
2024-2028 | 1 |  |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |
Funded status — STPNOC benefit plans | (83) | (128) |
Net periodic benefit costs |  |  | $ 3
Total recognized in other comprehensive loss | 21 | (15) |
Other Postretirement Benefit | South Texas Project |  |  |
Recognized amount in statement of financial position, statement of operations and accumulated OCI related to STP |  |  |
Funded status — STPNOC benefit plans | (19) | (24) |
Net periodic benefit costs | (7) | (3) |
Total recognized in other comprehensive loss | $ 2 | $ 5 |

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