REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 0.0	$ 4.2
Regulatory liabilities	1,777.4	1,703.7
Total regulatory liabilities	1,777.4	1,707.9
Income tax related items
Regulatory liabilities
Total regulatory liabilities	742.7	806.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	694.0	677.2
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	129.4	132.1
Derivatives
Regulatory liabilities
Total regulatory liabilities	106.6	5.9
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	64.1	61.7
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	25.8	15.5
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 14.8	$ 8.8