COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) (USD $) In Millions, unless otherwise specified		1 Months Ended
	Sep. 30, 2014 Manufactured gas plant remediation Natural Gas Utility site	Mar. 31, 2013 WPS Weston and Pulliam plants Electric Utility	Jun. 30, 2013 WPS Columbia and Edgewater jointly-owned plants Electric Utility	Sep. 30, 2014 WPS Weston Title V Air Permit Electric Utility	Sep. 30, 2014 WPS Mercury emission Electric Utility MW
Air Permitting Violation Claims
Beneficial environmental project amount		$ 6.0	$ 1.3
Civil penalty and/or legal fees		1.2	0.4
Number of claims to be withdrawn under the petition for review				9
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
Percentage mercury emission reduction from fuel combusted 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 of 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					9
Expended environmental capital costs to achieve required emission reduction					5
Manufactured Gas Plant Remediation
Number of environmental remediation sites	53
Number of environmental remediation sites transferred to the EPA Superfund Alternative Sites Program	20
Liabilities estimated and accrued for future undiscounted investigation and cleanup costs for all sites	557.9
Cash expenditures for environmental remediation not yet recovered in rates	56.6
Regulatory assets recorded for cash and estimated future remediation expenditures, net of insurance recoveries received	$ 614.5