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Pension and Other Postemployment Benefits (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 6 Months Ended
--- | --- | --- | ---
Jan. 31, 2018 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Additional funding contributions | $ 750 |  |  |  |
Pension |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Service costs |  | $ 56 | $ 52 | $ 112 | $ 104
Interest costs |  | 93 | 97 | 186 | 194
Expected return on plan assets |  | (144) | (112) | (288) | (224)
Amortization of prior service costs (credits) |  | 2 | 2 | 4 | 4
Net periodic costs (credits) |  | 7 | 39 | 14 | 78
Pension | FES and FENOC | Discontinued Operations, Disposed of by Means Other than Sale |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Net periodic costs (credits) |  | 13 | 15 | 26 | 31
OPEB |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Service costs |  | 1 | 1 | 2 | 2
Interest costs |  | 6 | 7 | 12 | 14
Expected return on plan assets |  | (7) | (7) | (15) | (15)
Amortization of prior service costs (credits) |  | (20) | (20) | (40) | (40)
Net periodic costs (credits) |  | (20) | (19) | (41) | (39)
OPEB | FES and FENOC | Discontinued Operations, Disposed of by Means Other than Sale |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Net periodic costs (credits) |  | (10) | (8) | (20) | (16)
FirstEnergy | Pension |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Net periodic benefit expense (credit) |  | (18) | 27 | (32) | 59
FirstEnergy | OPEB |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Net periodic benefit expense (credit) |  | $ (21) | $ (14) | $ (42) | $ (29)
Minimum |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |
Contributions by employer | $ 500 |  |  |  |

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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(ii) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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