REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Regulatory assets
Current assets	$ 25.0	$ 46.2
Long-term assets	433.5	342.5
Total regulatory assets	458.5	388.7
Other Disclosures
Environmental remediation liabilities	86.3	64.4
Unrecognized pension and other postretirement benefit costs
Regulatory assets
Total regulatory assets	185.6	130.6
Environmental remediation costs (net of insurance recoveries)
Regulatory assets
Total regulatory assets	103.8	80.1
Other Disclosures
Environmental remediation liabilities	86.3
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Total regulatory assets	44.6	50.0
Income tax related items
Regulatory assets
Total regulatory assets	32.7	26.9
Crane Creek production tax credits
Regulatory assets
Total regulatory assets	32.2	33.6
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	21.4	23.8
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	12.6	0
Asset retirement obligations
Regulatory assets
Total regulatory assets	5.6	6.0
Derivatives
Regulatory assets
Total regulatory assets	8.0	3.3
Potential new electric generator at Fox Energy Center
Regulatory assets
Total regulatory assets	3.7	0
Pension and other postretirement costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	0	9.4
Decoupling
Regulatory assets
Total regulatory assets	0	7.9
Weston 3 lightning strike
Regulatory assets
Total regulatory assets	0	3.6
Other
Regulatory assets
Total regulatory assets	$ 8.3	$ 13.5