REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Regulatory liabilities
Other current liabilities	$ 1.0	$ 3.6
Regulatory liabilities	777.1	772.6
Total regulatory liabilities	778.1	776.2
Income tax related items
Regulatory liabilities
Total regulatory liabilities	363.4	397.0
Removal costs
Regulatory liabilities
Total regulatory liabilities	192.4	193.7
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	115.4	115.9
Derivatives
Regulatory liabilities
Total regulatory liabilities	59.2	3.1
Earnings sharing mechanism
Regulatory liabilities
Total regulatory liabilities	25.1	36.8
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	19.5	16.8
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	1.0	9.7
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 2.1	$ 3.2