COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Mar. 31, 2013 Weston and Pulliam plants Electric Utility	Jun. 30, 2013 Columbia and Edgewater jointly-owned plants Electric Utility	Mar. 31, 2014 Mercury emission Electric Utility MW	Mar. 31, 2014 Manufactured gas plant remediation Natural Gas Utility site
Air Permitting Violation Claims
Beneficial environmental project amount	$ 6.0	$ 1.3
Civil penalty and/or legal fees	1.2	0.4
Mercury and Interstate Air Quality Rules
Percentage mercury reduction required by the State of Wisconsin's mercury rule through 2014			40.00%
Level of electric generating units above which a 90% reduction in mercury emissions is required by 2015 (in MW)			150
Percentage mercury reduction by 2015 required by the State of Wisconsin's mercury rule, for electric generating units above 150 MW			90.00%
Level of electric generating units above which a reduction in mercury emissions is required by the Best Available Control Technology rule (in MW)			25
Highest level for electric generating units for which a reduction in mercury emissions is required by the Best Available Control Technology rule (in MW)			150
Estimated capital cost to achieve required emission reduction			8
Manufactured Gas Plant Remediation
Number of environmental remediation sites				10
Number of environmental remediation sites transferred to the EPA Superfund Alternative Sites Program				7
Liabilities estimated and accrued for future undiscounted investigation and cleanup costs for all sites				64.1
Cash expenditures for environmental remediation not yet recovered in rates				12.0
Regulatory assets recorded for cash and estimated future remediation expenditures, net of insurance recoveries received				$ 76.1