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Benefit Plans and Other Postretirement Benefits - Net Benefit Costs and Funded Status (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 14, 2018 | Dec. 31, 2017 | Jun. 14, 2017
Annual periodic pension cost |  |  |  |  |  |  |
Curtailment gain |  |  | $ 80 |  |  |  |
GenOn |  |  |  |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Add: Retained obligation in bankruptcy proceeding |  |  |  | $ (25) |  |  |
Pension liability retained |  |  |  | 92 |  |  |
Pension Benefits |  |  |  |  |  |  |
Annual periodic pension cost |  |  |  |  |  |  |
Service cost benefits earned | $ 23 | $ 26 | 30 |  |  |  |
Interest cost on benefit obligation | 44 | 43 | 43 |  |  |  |
Expected return on plan assets | (62) | (58) | (60) |  |  |  |
Amortization of unrecognized net loss | 0 | 4 | 2 |  |  |  |
Curtailment gain | 7 | 0 | 0 |  |  |  |
Net recognized in net periodic pension cost/(credit) and OCI | 49 | (11) |  |  |  |  |
Net periodic benefit cost |  |  | 15 |  |  |  |
Pension and other post retirement benefit obligations |  |  |  |  |  |  |
Benefit obligation at January 1 | 1,329 | 1,241 |  |  |  |  |
Service cost | 23 | 26 | 30 |  |  |  |
Interest cost | 44 | 43 | 43 |  |  |  |
Plan amendments | 17 | 0 |  |  |  |  |
Actuarial (gain)/loss | (95) | 77 |  |  |  |  |
Employee and retiree contributions | 0 | 0 |  |  |  |  |
Curtailment gain | (20) | 0 |  |  |  |  |
Benefit payments | (76) | (58) |  |  |  |  |
Benefit obligation at December 31 | 1,222 | 1,329 | 1,241 |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 1,104 | 953 |  |  |  |  |
Actual return on plan assets | (80) | 173 |  |  |  |  |
Employee and retiree contributions | 0 | 0 |  |  |  |  |
Employer contributions | 33 | 36 |  |  |  |  |
Benefit payments | (76) | (58) |  |  |  |  |
Fair value of plan assets at December 31 | 981 | 1,104 | 953 |  |  |  |
Funded status at December 31 — excess of obligation over assets |  |  |  | (241) |  | $ (225) |
Less: GenOn postretirement obligation | 1,329 | 1,241 | 1,241 | 1,222 |  | 1,329 |
Pension Benefits | GenOn |  |  |  |  |  |  |
Pension and other post retirement benefit obligations |  |  |  |  |  |  |
Benefit obligation at January 1 | 0 |  |  |  |  |  |
Benefit obligation at December 31 | 0 | 0 |  |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Less: GenOn postretirement obligation | 0 | 0 |  | 0 |  | 0 |
Add: Retained obligation in bankruptcy proceeding |  |  |  | 0 |  | 0 |
Pension Benefits | NRG |  |  |  |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Funded status at December 31 — excess of obligation over assets |  |  |  | (241) |  | (225) |
Other Postretirement Benefit |  |  |  |  |  |  |
Annual periodic pension cost |  |  |  |  |  |  |
Service cost benefits earned | 1 | 1 | 2 |  |  |  |
Interest cost on benefit obligation | 4 | 4 | 6 |  |  |  |
Amortization of unrecognized prior service credit | (10) | (9) | (5) |  |  |  |
Amortization of unrecognized net loss | 0 | (1) | 0 |  |  |  |
Curtailment gain | (10) | 0 | 0 |  |  |  |
Net recognized in net periodic pension cost/(credit) and OCI | (36) | 13 |  |  |  |  |
Net periodic benefit cost |  |  | 3 |  |  |  |
Pension and other post retirement benefit obligations |  |  |  |  |  |  |
Benefit obligation at January 1 | 128 | 128 |  |  |  |  |
Service cost | 1 | 1 | 2 |  |  |  |
Interest cost | 4 | 4 | 6 |  |  |  |
Plan amendments | (28) | (1) |  |  |  |  |
Actuarial (gain)/loss | (6) | 6 |  |  |  |  |
Employee and retiree contributions | 3 | 3 |  |  |  |  |
Curtailment gain | (7) | 0 |  |  |  |  |
Benefit payments | (12) | (13) |  |  |  |  |
Benefit obligation at December 31 | 83 | 128 | 128 |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Fair value of plan assets at January 1 | 0 | 0 |  |  |  |  |
Actual return on plan assets | 0 | 0 |  |  |  |  |
Employee and retiree contributions | 3 | 3 |  |  |  |  |
Employer contributions | 9 | 10 |  |  |  |  |
Benefit payments | (12) | (13) |  |  |  |  |
Fair value of plan assets at December 31 | 0 | 0 | 0 |  |  |  |
Funded status at December 31 — excess of obligation over assets |  |  |  | (83) |  | (128) |
Less: GenOn postretirement obligation | 128 | 128 | $ 128 | 83 |  | 128 |
Other Postretirement Benefit | GenOn |  |  |  |  |  |  |
Pension and other post retirement benefit obligations |  |  |  |  |  |  |
Benefit obligation at January 1 | 38 |  |  |  |  |  |
Benefit obligation at December 31 | 0 | 38 |  |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Less: GenOn postretirement obligation | $ 38 | $ 38 |  | 0 | $ 23 | 38 |
Add: Retained obligation in bankruptcy proceeding |  |  |  | 0 |  | (25) | $ (25)
Other Postretirement Benefit | NRG |  |  |  |  |  |  |
Fair value of plan assets for pension and other post retirement benefit |  |  |  |  |  |  |
Funded status at December 31 — excess of obligation over assets |  |  |  | $ (83) |  | $ (115) |

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- Definition Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 + Details
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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=112275985&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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