Regulatory Matters - Pennsylvania and West Virginia (Details) $ in Millions				1 Months Ended	9 Months Ended
	Aug. 27, 2021 USD ($)	Dec. 30, 2020 USD ($)	Jan. 16, 2020 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2021 USD ($) proposal	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 | Pennsylvania Companies | PPUC | EE&C Phase III
Regulatory Matters [Line Items]
Approved amount of annual increase				$ 390.0
Pennsylvania | Pennsylvania Companies | PPUC | New LTIIPs
Regulatory Matters [Line Items]
Amount of requested rate increase (decrease)			$ 572.0
Recovery period			5 years
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 | ENEC
Regulatory Matters [Line Items]
Approved amount of annual increase		$ (7.7)
Amount of requested rate increase (decrease)	$ 19.6	$ (2.6)
Amount of requested rate increase (decrease) (percent)	1.50%
Supplemental requested decrease	$ (7.7)