REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Regulatory liabilities
Current liabilities	$ 13.7	$ 7.2
Regulatory liabilities	782.5	785.7
Total regulatory liabilities	796.2	792.9
Income tax related items
Regulatory liabilities
Total regulatory liabilities	416.7	418.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	201.8	241.8
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	100.5	72.6
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	42.0	21.2
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	20.0	14.3
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	3.7	9.7
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 11.5	$ 14.5