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LEASES -Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($)
Lessor, Lease, Description [Line Items] |
Maximum potential loss of lease agreement | $ 16
Amount of leases not yet commenced | $ 14
Expected commencement period | 18 months
Minimum |
Lessor, Lease, Description [Line Items] |
Renewal term of lease yet to be commenced | 1 year
Operating lease renewal term | 5 years
Maximum |
Lessor, Lease, Description [Line Items] |
Renewal term of lease yet to be commenced | 40 years
Operating lease renewal term | 10 years

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- Definition Lessee, Operating Lease, Lease Not yet Commenced, Amount + References No definition available. + Details
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Namespace Prefix: | fe_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Lessee, Operating Lease, Lease Not yet Commenced, Expected Commencement Period + References No definition available. + Details
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Namespace Prefix: | fe_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Lessee, Operating Lease, Residual Value Guarantee, Potential Loss + References No definition available. + Details
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Namespace Prefix: | fe_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Term of lessee's operating lease renewal for lease not yet commenced, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

X
- Definition Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918673-209980 + Details
Name: | us-gaap_LesseeOperatingLeaseRenewalTerm
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_LessorLeaseDescriptionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |