REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Regulatory liabilities
Other current liabilities	$ 12.6	$ 8.4
Regulatory liabilities	732.0	735.5
Total regulatory liabilities	744.6	743.9
Income tax related items
Regulatory liabilities
Total regulatory liabilities	347.6	352.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	191.7	189.9
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	116.2	116.5
Derivatives
Regulatory liabilities
Total regulatory liabilities	36.0	26.9
Earnings sharing mechanism
Regulatory liabilities
Total regulatory liabilities	20.7	26.7
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	15.1	19.7
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	12.6	8.4
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 4.7	$ 3.1