Segment Information - Narrative (Details) mi² in Thousands, customer in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) mi² customer company MW	Dec. 31, 2020 USD ($)
Regulated Distribution
Segment Reporting Information [Line Items]
Number of existing utility operating companies | company	10
Number of customers served by utility operating companies | customer	6
Number of square miles in service area | mi²	65
Megawatts of net demonstrated capacity of competitive segment | MW	3,580
Other/Corporate | OVEC
Segment Reporting Information [Line Items]
Megawatts of net demonstrated capacity of competitive segment | MW	67
FirstEnergy | Other/Corporate
Segment Reporting Information [Line Items]
Long-term debt and other long-term obligations | $	$ 7,400
Held-for-sale | Yard Creek Generating Facility | Regulated Distribution
Segment Reporting Information [Line Items]
Investments - held for sale | $		$ 45