REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory liabilities
Current liabilities	$ 9.6	$ 7.2
Regulatory liabilities	758.7	785.7
Total regulatory liabilities	768.3	792.9
Income tax related items
Regulatory liabilities
Total regulatory liabilities	416.0	418.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	233.4	241.8
Pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	71.7	72.6
Earnings sharing mechanism
Regulatory liabilities
Total regulatory liabilities	21.3	21.2
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	16.7	14.3
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	6.5	9.7
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 2.7	$ 14.5