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FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 05, 2024 | Sep. 03, 2024 | Dec. 31, 2023
Fair Value of Financial Instruments [Line Items] |  |  |  |  |  |  |
Investments not required to be disclosed | $ 402 |  | $ 402 |  |  |  | $ 382
Senior Notes | FET |  |  |  |  |  |  |
Fair Value of Financial Instruments [Line Items] |  |  |  |  |  |  |
Face amount of debt |  |  |  |  | $ 800 |  |
Effective shelf registration, effective period |  |  | 60 days |  |  |  |
Interest accrual rate for the first 90 days |  |  | 0.25% |  |  |  |
Additional interest accrual rate for subsequent 90 days |  |  | 0.25% |  |  |  |
Additional interest accrual maximum rate per year |  |  | 0.50% |  |  |  |
5.17%, $200 Million Unsecured Notes | Senior Notes | KATCo |  |  |  |  |  |  |
Fair Value of Financial Instruments [Line Items] |  |  |  |  |  |  |
Face amount of debt |  |  |  |  |  | $ 200 |
Issuance interest rate |  |  |  |  |  | 5.17% |
Corporate-Owned Life Insurance |  |  |  |  |  |  |
Fair Value of Financial Instruments [Line Items] |  |  |  |  |  |  |
Gain (loss) on investments | $ 10 | $ (4) | $ 20 | $ 6 |  |  |

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