REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Regulatory liabilities
Current liabilities	$ 0.0	$ 7.9
Regulatory liabilities	705.4	689.3
Total regulatory liabilities	705.4	697.2
Income tax
Regulatory liabilities
Total regulatory liabilities	395.0	393.8
Removal costs
Regulatory liabilities
Total regulatory liabilities	238.8	238.9
Unrecognized pension and OPEB costs
Regulatory liabilities
Total regulatory liabilities	29.0	30.2
Electric transmission costs
Regulatory liabilities
Total regulatory liabilities	9.9	6.0
Earnings sharing mechanisms
Regulatory liabilities
Total regulatory liabilities	8.5	0.0
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	6.2	8.2
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 18.0	$ 20.1