REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 6.7	$ 51.0
Regulatory liabilities	4,131.2	3,928.1
Total regulatory liabilities	4,137.9	3,979.1
Income tax related items
Regulatory liabilities
Total regulatory liabilities	2,030.1	2,137.7
Removal costs
Regulatory liabilities
Total regulatory liabilities	1,244.5	1,221.1
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	366.3	378.1
Derivatives
Regulatory liabilities
Total regulatory liabilities	298.7	16.4
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	83.5	78.5
Uncollectible expense
Regulatory liabilities
Total regulatory liabilities	47.2	25.5
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	25.2	36.9
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	5.5	59.9
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 36.9	$ 25.0