REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Regulatory liabilities
Other current liabilities	$ 96.8	$ 14.3
Regulatory liabilities	4,032.6	3,946.0
Total regulatory liabilities	4,129.4	3,960.3
Income tax related items
Regulatory liabilities
Total regulatory liabilities	1,981.6	1,998.5
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,247.2	1,248.0
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	392.8	397.3
Derivatives
Regulatory liabilities
Total regulatory liabilities	245.3	124.1
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	97.6	13.7
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	65.5	84.2
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	29.7	37.1
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	22.5	28.4
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 47.2	$ 29.0