REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Regulatory assets
Total regulatory assets	$ 382.8	$ 465.8
Other Disclosures
Regulatory assets not earning a return	38.0
Regulatory assets earning a return based on short-term rates	5.3
Environmental remediation liabilities	99.6	97.2
Unrecognized pension and OPEB costs
Regulatory assets
Total regulatory assets	161.3	182.9
Environmental remediation costs
Regulatory assets
Total regulatory assets	116.0	116.0
Other Disclosures
Environmental remediation liabilities	99.6
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Total regulatory assets	27.2	33.7
Crane Creek production tax credits
Regulatory assets
Total regulatory assets	$ 22.8	29.6
Other Disclosures
Period over which grant received will be reflected in income through reduction of depreciation and amortization expense	12 years
De Pere Energy Center
Regulatory assets
Total regulatory assets	$ 14.0	16.7
Income tax related items
Regulatory assets
Total regulatory assets	8.2	55.7
Other, net
Regulatory assets
Total regulatory assets	$ 33.3	$ 31.2