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	$ 118.9	$ 127.4
Long-term assets	1,513.6	1,361.4
Total regulatory assets	1,632.5	1,488.8
Other disclosures
Environmental remediation liabilities	579.9	600.0
Environmental remediation costs (net of insurance recoveries)
Regulatory assets
Total regulatory assets	635.8	652.1
Other disclosures
Environmental remediation liabilities	579.9
Unrecognized pension and other postretirement benefit costs
Regulatory assets
Total regulatory assets	513.1	382.6
Asset retirement obligations
Regulatory assets
Total regulatory assets	109.4	89.0
Merger and acquisition-related pension and other postretirement benefit costs
Regulatory assets
Total regulatory assets	86.6	98.3
Income tax related items
Regulatory assets
Total regulatory assets	60.6	55.3
Derivatives
Regulatory assets
Total regulatory assets	55.2	11.7
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Total regulatory assets	44.6	50.0
Crane Creek production tax credits
Regulatory assets
Total regulatory assets	32.2	33.6
Other disclosures
Period over which the grant received will be reflected in income through a reduction of depreciation and amortization expense	12 years
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	22.2	17.0
De Pere Energy Center
Regulatory assets
Total regulatory assets	21.4	23.8
Unamortized loss on reacquired debt
Regulatory assets
Total regulatory assets	16.6	16.2
Uncollectible expense
Regulatory assets
Total regulatory assets	13.6	4.7
Energy efficiency programs
Regulatory assets
Total regulatory assets	2.8	16.8
Pension and other postretirement costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	0	9.4
Decoupling
Regulatory assets
Total regulatory assets	0	8.6
Other
Regulatory assets
Total regulatory assets	$ 18.4	$ 19.7