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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

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