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