REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Regulatory liabilities
Other current liabilities	$ 79.2	$ 47.5
Regulatory liabilities	3,730.2	3,697.7
Total regulatory liabilities	3,809.4	3,745.2
Income tax related items
Regulatory liabilities
Total regulatory liabilities	1,862.6	1,901.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,365.4	1,329.9
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	300.0	299.2
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	119.3	72.4
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	31.0	30.3
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	20.6	17.2
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	19.0	21.2
Derivatives
Regulatory liabilities
Total regulatory liabilities	15.2	19.2
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 76.3	$ 54.0