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REGULATORY MATTERS - Reliability and FERC Matters (Details) - USD ($) $ in Millions |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Feb. 02, 2021 | Nov. 01, 2020 | Dec. 31, 2020 | Dec. 31, 2019
ATSI | FERC |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Regulatory asset balance |  |  | $ 79 | $ 73
ATSI | Regulated Transmission |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved ROE |  |  | 10.38% |
MAIT | Regulated Transmission |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Allowed Debt |  |  | 60.00% |
Approved ROE |  |  | 10.30% |
TrAIL | Regulated Transmission | TrAIL the Line and Black Oak SVC |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved ROE |  |  | 12.70% |
TrAIL | Regulated Transmission | All Other Projects |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved ROE |  |  | 11.70% |
JCP&L |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Allowed Debt |  | 48.56% | 55.00% |
Approved ROE |  | 9.60% | 9.60% |
JCP&L | Regulated Transmission |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved ROE |  |  | 10.80% |
JCP&L | Regulated Transmission | Subsequent Event |  |  |  |
Regulatory Matters [Line Items] |  |  |  |
Approved ROE | 10.20% |  |  |

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