REGULATORY ASSETS AND LIABILITIES (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Regulatory assets
Current assets	$ 129.4	$ 110.8
Long-term assets	1,412.6	1,813.8
Total regulatory assets	1,542.0	1,924.6
Other disclosures
Environmental remediation liabilities	600.8	651.5
Environmental remediation costs (net of insurance recoveries)
Regulatory assets
Total regulatory assets	653.0	689.7
Other disclosures
Environmental remediation liabilities	600.8
Unrecognized pension and other postretirement benefit costs
Regulatory assets
Total regulatory assets	428.8	810.0
Merger and acquisition related pension and other postretirement benefit costs
Regulatory assets
Total regulatory assets	98.3	110.1
Asset retirement obligations
Regulatory assets
Total regulatory assets	89.5	73.9
Income tax related items
Regulatory assets
Total regulatory assets	55.3	47.6
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 the 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	23.8	26.2
Unamortized loss on reacquired debt
Regulatory assets
Total regulatory assets	18.5	18.2
Energy efficiency programs
Regulatory assets
Total regulatory assets	18.0	16.7
Energy costs receivable through rate adjustments
Regulatory assets
Total regulatory assets	16.0	18.1
Derivatives
Regulatory assets
Total regulatory assets	11.8	30.7
Pension and other postretirement costs receivable through rate adjustments
Regulatory assets
Total regulatory assets	9.4	0
Decoupling
Regulatory assets
Total regulatory assets	9.3	10.6
Other
Regulatory assets
Total regulatory assets	$ 26.7	$ 37.9