VARIABLE INTEREST ENTITIES - TRANSMISSION AFFILIATES (Details) - USD ($) $ in Millions	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
ATC
Variable interest entities
Ownership interest (as a percent)	60.00%
Equity investment	$ 2,029.7	$ 2,001.6	$ 1,980.8	$ 1,931.8	$ 1,896.2	$ 1,884.6
ATC Holdco
Variable interest entities
Ownership interest (as a percent)	75.00%
Equity investment	$ 26.1	$ 25.5	$ 25.1	$ 24.1	$ 25.5	$ 24.6