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Discontinued Operations - Narrative (Details) $ in Millions |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
May 03, 2018 | Mar. 16, 2018 USD ($) | Mar. 09, 2018 USD ($) MW | Sep. 30, 2019 USD ($) MW | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) MW | Dec. 31, 2018 USD ($) MW | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) MW | Mar. 31, 2018 USD ($) | Aug. 31, 2017 MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term borrowings |  |  |  | $ 1,000.0 |  | $ 1,000.0 | $ 1,250.0 |  | $ 1,250.0 |  |
Interest accrued |  |  |  | 261.0 | $ 255.0 | 773.0 |  | $ 858.0 |  |  |
Outstanding guarantees net liability |  |  |  | 1,600.0 |  | 1,600.0 |  |  |  |  |
Future worthless stock deduction |  |  |  |  |  | 4,700.0 |  |  |  |  |
Future worthless stock deduction, net of tax |  |  |  |  |  | 1,000.0 |  |  |  |  |
Unrecognized tax benefits from worthless stock deduction |  |  |  |  |  | 448.0 |  |  |  |  |
Unrecognized tax benefits from worthless stock deduction, net of tax |  |  |  |  |  | 94.0 |  |  |  |  |
Current assets - discontinued operations |  |  |  | $ 33.0 |  | $ 33.0 | $ 25.0 |  | $ 25.0 |  |
Purchase Agreement with Subsidiary of LS Power |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  |  |  |  |  |  |  |  | 1,615
AGC | Purchase Agreement with Subsidiary of LS Power |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership percentage | 59.00% |  |  |  |  |  |  |  |  |  |
Pleasants Power Station | AE Supply | Purchase Agreement with Subsidiary of LS Power |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  |  | 1,300 |  | 1,300 | 1,300 |  | 1,300 |  |
Bay Shore Unit 1 | Asset Purchase Agreement with Walleye Energy, LLC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Plant capacity (in MW's) | MW |  |  | 136 |  |  |  |  |  |  |  |
Promissory Notes | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest accrued |  |  |  |  |  | $ 26.0 |  | 16.0 |  |  |
Promissory Notes | AE Supply | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Outstanding borrowings |  |  |  | $ 102.0 | 102.0 | 102.0 |  | 102.0 |  |  |
Revolving Credit Facility | Line of Credit | FES |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Line of credit outstanding |  |  | $ 500.0 | 500.0 |  | 500.0 |  |  |  |  |
Loan reserves |  |  |  | 500.0 |  |  |  |  |  |  |
Money Pool | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term borrowings |  |  |  |  | 92.0 |  |  | 92.0 |  |  |
Reinstated debt under settlement agreement |  | $ 88.0 |  |  |  |  |  |  |  |  |
Money Pool | Adjustment | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Short-term borrowings |  | $ 92.0 |  |  |  |  |  |  |  |  |
Discontinued Operations, Disposed of by Means Other than Sale | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investments in subsidiaries |  |  |  | 0.0 |  | 0.0 |  |  |  |  |
Gain (loss) on disposal of FES and FENOC, net of tax |  |  |  | 23.0 | (834.0) | (16.0) |  | 405.0 | $ 435.0 |  |
Affiliated Companies | Discontinued Operations, Disposed of by Means Other than Sale | FES | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Effects of Tax Act of included in income tax expense from discontinued operations |  |  |  | 17.0 | 12.0 | 45.0 |  | 48.0 |  |  |
Services Agreements | Affiliated Companies | Discontinued Operations, Disposed of by Means Other than Sale | FES | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amounts of transaction |  |  |  | 20.0 |  | 121.0 | $ 112.5 |  |  |  |
Benefit Obligations | Affiliated Companies | Discontinued Operations, Disposed of by Means Other than Sale | FES | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amounts of transaction |  |  |  | 9.0 |  | 28.0 |  |  |  |  |
Power Purchase Agreements | Affiliated Companies | Discontinued Operations, Disposed of by Means Other than Sale | FES | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Due from related parties |  |  |  | 6.0 |  | 6.0 |  |  |  |  |
Purchases from related party |  |  |  | 24.0 | 74.0 | $ 150.0 |  | $ 248.0 |  |  |
FE | Promissory Notes | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loan reserves |  |  |  | $ 9.0 | 102.0 |  |  |  |  |  |
FE | Unregulated Money Pool | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loan reserves |  |  |  |  | $ 92.0 |  |  |  |  |  |
Financial Guarantee | Benefit Obligations | Affiliated Companies | Discontinued Operations, Disposed of by Means Other than Sale | FES and FENOC |  |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |  |
Outstanding guarantees net liability |  |  |  |  |  |  |  |  |  | $ 820.0 |

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