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SEGMENT INFORMATION - Narrative (Details) mi² in Thousands, customer in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) mi² company MW	Dec. 31, 2020 USD ($) mi² customer company MW
Utilities and FET
Segment Reporting Information [Line Items]
Number of existing utility operating companies | company	10	10
Number of customers served by utility operating companies | customer		6
Number of square miles in service area | mi²	65	65
Megawatts of net demonstrated capacity of competitive segment (in MW's) | MW	3,790	3,790
Utilities and FET | Disposal Group, Held-for-sale | TMI-2
Segment Reporting Information [Line Items]
Assets held-for-sale	$ 882	$ 882
Utilities and FET | Disposal Group, Held-for-sale | Yard Creek Generating Facility
Segment Reporting Information [Line Items]
Assets held-for-sale	45	$ 45
Utilities and FET | Disposal Group, Disposed of by Sale | TMI-2
Segment Reporting Information [Line Items]
Gain on disposal of discontinued operation, net of tax	$ 33
Utilities and FET | Yard's Creek Energy, LLC Hydro Generation Facility
Segment Reporting Information [Line Items]
Megawatts of net demonstrated capacity of competitive segment (in MW's) | MW	210	210
Ownership interest acquired		50.00%
Other/Corporate | OVEC
Segment Reporting Information [Line Items]
Megawatts of net demonstrated capacity of competitive segment (in MW's) | MW	67	67
FE | Other/Corporate
Segment Reporting Information [Line Items]
Long-term debt and other long-term obligations	$ 8,200	$ 8,200

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Amount classified as assets attributable to disposal group held for sale or disposed of.

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