Regulatory Matters - Pennsylvania and West Virginia (Details) $ in Millions											3 Months Ended
	Mar. 17, 2022 USD ($)	Mar. 02, 2022 USD ($)	Dec. 29, 2021 USD ($)	Dec. 27, 2021 USD ($)	Dec. 17, 2021 USD ($)	Nov. 22, 2021 MW	Nov. 18, 2021 USD ($)	Aug. 27, 2021 USD ($)	Dec. 30, 2020 USD ($)	Jan. 16, 2020 USD ($)	Mar. 31, 2022 USD ($) proposal	Sep. 30, 2021 USD ($)	Jun. 18, 2020
FERC | Transmission Related Vegetation Management Programs | Other Nonoperating Income (Expense) | FirstEnergy
Regulatory Matters [Line Items]
Pre-tax impairment of regulatory asset												$ 21.0
ATSI | FERC | Transmission Related Vegetation Management Programs | Regulated Transmission
Regulatory Matters [Line Items]
Pre-tax impairment of regulatory asset												48.0
ATSI | FERC | Transmission Related Vegetation Management Programs | Regulated Distribution
Regulatory Matters [Line Items]
Pre-tax impairment of regulatory asset												$ 27.0
Pennsylvania | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Number of RFP's | proposal											2
RFP term											2 years
Pennsylvania | Three month period | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Term of energy contract											3 months
Pennsylvania | Twelve month period | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Term of energy contract											12 months
Pennsylvania | Twenty-four month period | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Term of energy contract											24 months
Pennsylvania | PPUC
Regulatory Matters [Line Items]
Amended amount of rate increase							$ 61.0
Pennsylvania | PPUC | Regulated Distribution
Regulatory Matters [Line Items]
Pre-tax charges											$ 61.0
Pennsylvania | Pennsylvania Companies | PPUC | New LTIIPs
Regulatory Matters [Line Items]
Recovery period										5 years
Amount of requested rate increase (decrease)										$ 572.0
Pennsylvania | ME | PPUC | EE&C Phase IV
Regulatory Matters [Line Items]
Demand reduction targets													2.90%
Energy consumption reduction targets													3.10%
Pennsylvania | PN | PPUC | EE&C Phase IV
Regulatory Matters [Line Items]
Demand reduction targets													3.30%
Energy consumption reduction targets													3.00%
Pennsylvania | Penn | PPUC | EE&C Phase IV
Regulatory Matters [Line Items]
Demand reduction targets													2.00%
Energy consumption reduction targets													2.70%
Pennsylvania | WP | PPUC | EE&C Phase IV
Regulatory Matters [Line Items]
Demand reduction targets													2.50%
Energy consumption reduction targets													2.40%
West Virginia | MP and PE | WVPSC
Regulatory Matters [Line Items]
Amount of requested rate increase (decrease)		$ 94.0
West Virginia | MP and PE | WVPSC | Ft. Martin and Harrison Power Stations
Regulatory Matters [Line Items]
Amount of requested rate increase (decrease)					$ 142.0
Requested annual rate increase					$ 3.0
West Virginia | MP and PE | WVPSC | Solar Generation Project
Regulatory Matters [Line Items]
Plant capacity (in MW's) | MW						50
Expected cost of the program	$ 110.0
West Virginia | MP and PE | WVPSC | ENEC
Regulatory Matters [Line Items]
Amount of requested rate increase (decrease)								$ 19.6	$ (2.6)
Approved amount of annual increase									$ (7.7)
Amount of requested rate increase (decrease) (percent)								1.50%
Supplemental requested decrease								$ (7.7)
Expected cost of the program			$ 2.9
West Virginia | MP and PE | WVPSC | Integrated Resource Plan
Regulatory Matters [Line Items]
Approved amount of annual increase			$ 19.6
West Virginia | MP and PE | WVPSC | VMS
Regulatory Matters [Line Items]
Approved amount of annual increase				$ 16.0
Surcharge period				2 years