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Benefit Plans and Other Postretirement Benefits - Amounts Recognized in the Balance Sheets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Benefits |  |  |
Amounts recognized in balance sheet |  |  |
Current liabilities | $ 0 | $ 0 |
Non-current liabilities | 225 | 288 |
Amounts recognized in accumulated OCI |  |  |
Net loss/(gain) | (53) | (94) |
Prior service cost/(credit) | 3 | 3 |
Total accumulated OCI | 56 | 97 |
Other changes in plan assets and benefit obligations recognized in other comprehensive income |  |  |
Net actuarial (gain)/loss | (37) | 28 |
Amortization of net actuarial (gain)/loss | (4) | (2) | $ (2)
Prior service credit | 0 | 0 |
Amortization of prior service cost | 0 | 0 |
Total recognized in other comprehensive loss | (41) | 26 |
Net recognized in net periodic pension (credit)/cost and OCI | (11) | 24 |
Estimated unrecognized loss for NRG's pension plan that will be amortized from accumulated OCI to net periodic cost over the next fiscal year (less than) | 1 |  |
Significant components of NRG's domestic pension plan |  |  |
Projected benefit obligation | 1,329 | 1,241 | 1,196
Accumulated benefit obligation | 1,255 | 1,174 |
Fair value of plan assets | 1,104 | 953 | 916
Other Postretirement Benefit |  |  |
Amounts recognized in balance sheet |  |  |
Current liabilities | 7 | 8 |
Non-current liabilities | 121 | 120 |
Amounts recognized in accumulated OCI |  |  |
Net loss/(gain) | 4 | 11 |
Prior service cost/(credit) | (37) | (45) |
Total accumulated OCI | (41) | (56) |
Other changes in plan assets and benefit obligations recognized in other comprehensive income |  |  |
Net actuarial (gain)/loss | 6 | (2) |
Amortization of net actuarial (gain)/loss | 1 | 0 | (1)
Prior service credit | (1) | (41) |
Amortization of prior service cost | 9 | 5 |
Total recognized in other comprehensive loss | 15 | (38) |
Net recognized in net periodic pension (credit)/cost and OCI | 13 | 39 |
Expected unrecognized gain that will be amortized from accumulated OCI to net periodic cost over the next fiscal year | 1 |  |
Expected amortization of unrecognized prior service credit over the next fiscal Year |  | 7 |
Significant components of NRG's domestic pension plan |  |  |
Projected benefit obligation | 128 | 128 | 178
Fair value of plan assets | 0 | 0 | $ 0
GenOn |  |  |
Amounts recognized in balance sheet |  |  |
Pension benefit obligations | 92 | 120 |
Liability retained other post-employment and retiree health and welfare benefits | 25 |  |
Amounts recognized in accumulated OCI |  |  |
Net loss/(gain) | 28 |  |
GenOn | Pension Benefits |  |  |
Amounts recognized in balance sheet |  |  |
Current liabilities | 0 | 0 |
Non-current liabilities | 0 | 0 |
Liability retained other post-employment and retiree health and welfare benefits | 0 | 0 |
Significant components of NRG's domestic pension plan |  |  |
Projected benefit obligation | 0 | 0 |
GenOn | Other Postretirement Benefit |  |  |
Amounts recognized in balance sheet |  |  |
Current liabilities | 3 | 5 |
Non-current liabilities | 10 | 16 |
Pension benefit obligations | 13 | 21 |
Liability retained other post-employment and retiree health and welfare benefits | 25 | 25 |
Significant components of NRG's domestic pension plan |  |  |
Projected benefit obligation | 38 | 46 |
NRG | Pension Benefits |  |  |
Amounts recognized in balance sheet |  |  |
Current liabilities | 0 | 0 |
Non-current liabilities | 225 | 288 |
NRG | Other Postretirement Benefit |  |  |
Amounts recognized in balance sheet |  |  |
Current liabilities | 4 | 3 |
Non-current liabilities | 111 | 104 |
Discontinued Operations | Pension Benefits |  |  |
Amounts recognized in accumulated OCI |  |  |
Total accumulated OCI | (22) | (37) |
Other changes in plan assets and benefit obligations recognized in other comprehensive income |  |  |
Total recognized in other comprehensive loss | 15 | (17) |
Net recognized in net periodic pension (credit)/cost and OCI | 15 | (17) |
Discontinued Operations | Other Postretirement Benefit |  |  |
Amounts recognized in accumulated OCI |  |  |
Total accumulated OCI | 10 | 8 |
Other changes in plan assets and benefit obligations recognized in other comprehensive income |  |  |
Total recognized in other comprehensive loss | 2 | 3 |
Net recognized in net periodic pension (credit)/cost and OCI | 3 | 3 |
Continuing Operations | Pension Benefits |  |  |
Amounts recognized in accumulated OCI |  |  |
Total accumulated OCI | 34 | 60 |
Other changes in plan assets and benefit obligations recognized in other comprehensive income |  |  |
Total recognized in other comprehensive loss | (26) | 9 |
Continuing Operations | Other Postretirement Benefit |  |  |
Amounts recognized in accumulated OCI |  |  |
Total accumulated OCI | (31) | (48) |
Other changes in plan assets and benefit obligations recognized in other comprehensive income |  |  |
Total recognized in other comprehensive loss | $ 17 | $ (35) |

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