VARIABLE INTEREST ENTITIES - TRANSMISSION AFFILIATES (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
ATC
Variable interest entities
Ownership interest (as a percent)	60.00%
Equity investment	$ 1,959.2	$ 1,931.8	$ 1,884.6	$ 1,851.8	$ 1,813.6	$ 1,766.9
ATC Holdco
Variable interest entities
Ownership interest (as a percent)	75.00%
Equity investment	$ 24.6	$ 24.1	$ 24.6	$ 24.1	$ 23.6	$ 22.5