Discontinued Operations - Narrative (Details) $ in Millions				1 Months Ended	3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	May 03, 2018	Mar. 16, 2018 USD ($)	Mar. 09, 2018 USD ($) MW	Aug. 31, 2017 Natural_gas_plant MW	Jun. 30, 2019 USD ($) MW	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) MW	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) MW	Dec. 31, 2018 USD ($) MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Short-term borrowings					$ 1,250.0		$ 1,250.0		$ 1,250.0	$ 1,250.0
Interest accrued					259.0	$ 355.0	512.0	$ 603.0
Outstanding guarantees net liability					1,700.0		1,700.0
Future worthless stock deduction							4,800.0
Future worthless stock deduction, net of tax							1,000.0
Unrecognized tax benefits from worthless stock deduction							418.0
Unrecognized tax benefits from worthless stock deduction, net of tax							88.0
Current assets - discontinued operations					$ 44.0		$ 44.0		$ 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
AE Supply | Purchase Agreement with Subsidiary of LS Power
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of gas generating plants | Natural_gas_plant				4
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%			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							$ 18.0	8.0
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					(15.0)	0.0	(39.0)	1,239.0		$ 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					14.0	20.0	28.0	36.0
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					67.0		101.0		$ 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		19.0
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					7.0		7.0
Purchases from related party					43.0	71.0	$ 126.0	174.0
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		$ 820.0