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