REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 23.3	$ 51.0
Regulatory liabilities	3,935.5	3,928.1
Total regulatory liabilities	3,958.8	3,979.1
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,071.6	2,137.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,243.6	1,221.1
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	367.7	378.1
Derivatives
Regulatory liabilities
Total regulatory liabilities	95.6	16.4
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	78.4	78.5
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	35.6	59.9
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	28.3	36.9
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	12.2	25.5
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 25.8	$ 25.0