REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 2.9	$ 4.2
Regulatory liabilities	1,700.8	1,703.7
Total regulatory liabilities	1,703.7	1,707.9
Income tax related items
Regulatory liabilities
Total regulatory liabilities	768.7	806.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	689.3	677.2
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	129.1	132.1
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	61.4	61.7
Derivatives
Regulatory liabilities
Total regulatory liabilities	36.1	5.9
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 19.1	$ 24.3