REGULATORY ASSETS AND LIABILITIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Regulatory assets
Current assets	$ 46.2	$ 26.1
Long-term assets	342.5	536.2
Total regulatory assets	388.7	562.3
Other Disclosures
Environmental remediation liabilities	64.4	68.8
Unrecognized pension and other postretirement benefit costs
Regulatory assets
Total regulatory assets	130.6	346.7
Environmental remediation costs (net of insurance recoveries)
Regulatory assets
Total regulatory assets	80.1	83.0
Other Disclosures
Environmental remediation liabilities	64.4
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Total regulatory assets	50.0	0
Crane Creek production tax credits
Regulatory assets
Total regulatory assets	33.6	34.9
Other Disclosures
Period over which grant received will be reflected in income through reduction of depreciation and amortization expense	12 years
Income tax related items
Regulatory assets
Total regulatory assets	26.9	19.7
De Pere Energy Center
Regulatory assets
Total regulatory assets	23.8	26.2
Pension and other postretirement costs receivable through rate adjustments
Regulatory assets
Total regulatory assets	9.4	0
Decoupling
Regulatory assets
Total regulatory assets	7.9	7.9
Asset retirement obligations
Regulatory assets
Total regulatory assets	6.0	6.5
Weston 3 lightning strike
Regulatory assets
Total regulatory assets	3.6	7.3
Derivatives
Regulatory assets
Total regulatory assets	3.3	10.5
Other
Regulatory assets
Total regulatory assets	$ 13.5	$ 19.6