REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Regulatory liabilities
Other current liabilities	$ 69.6	$ 56.4
Regulatory liabilities	3,720.0	3,735.5
Total regulatory liabilities	3,789.6	3,791.9
Income tax related items
Regulatory liabilities
Total regulatory liabilities	1,916.8	1,956.6
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,308.2	1,260.9
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	327.2	340.5
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	84.3	53.4
Derivatives
Regulatory liabilities
Total regulatory liabilities	33.9	76.7
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	26.9	24.0
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	21.0	0.4
Decoupling
Regulatory liabilities
Total regulatory liabilities	14.9	20.2
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	8.1	10.4
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 48.3	$ 48.8