REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 8.4	$ 3.6
Regulatory liabilities	735.5	772.6
Total regulatory liabilities	743.9	776.2
Income tax related items
Regulatory liabilities
Total regulatory liabilities	352.7	397.0
Removal costs
Regulatory liabilities
Total regulatory liabilities	189.9	193.7
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	116.5	115.9
Derivatives
Regulatory liabilities
Total regulatory liabilities	26.9	3.1
Earnings sharing mechanism
Regulatory liabilities
Total regulatory liabilities	26.7	36.8
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	19.7	16.8
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	8.4	9.7
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 3.1	$ 3.2