COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Mar. 31, 2013	Jun. 30, 2013	Oct. 31, 2014	Dec. 31, 2014 MW
Weston and Pulliam plants | Electric Utility
Air Permitting Violation Claims
Beneficial environmental project amount	$ 6.0
Civil penalty and/or legal fees	1.2
Columbia and Edgewater jointly-owned plants | Electric Utility
Air Permitting Violation Claims
Beneficial environmental project amount		1.3
Civil penalty and/or legal fees		0.4
Mercury emission | Electric Utility
Mercury and Interstate Air Quality Rules
Percentage mercury emission 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 from fuel combusted is required by 2015 (in MW)				150
Estimated capital cost to achieve required emission reduction				9
Expended environmental capital costs to achieve required emission reduction				8
Sulfur dioxide and nitrogen oxide | Electric Utility
Mercury and Interstate Air Quality Rules
EPA change to compliance deadline for Cross State Air Pollution Rule			3 years
Manufactured gas plant remediation | Natural Gas Utility
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				86.3
Cash expenditures for environmental remediation not yet recovered in rates				16.0
Regulatory assets recorded for cash and estimated future remediation expenditures, net of insurance recoveries received				$ 102.3