REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 12.5	$ 51.0
Regulatory liabilities	3,890.8	3,928.1
Total regulatory liabilities	3,903.3	3,979.1
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,111.0	2,137.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,234.6	1,221.1
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	373.1	378.1
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	75.9	78.5
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	30.8	36.9
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	24.7	59.9
Derivatives
Regulatory liabilities
Total regulatory liabilities	17.4	16.4
Energy efficiency programs
Regulatory liabilities
Total regulatory liabilities	11.0	9.9
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	3.2	25.5
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 21.6	$ 15.1