COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2014 compliance_option	Mar. 31, 2018 USD ($) degreecelsius MW	Dec. 31, 2017 USD ($) T	Dec. 31, 2015	Sep. 30, 2016 Case
Climate Change | Electric
Air quality
Number of legal cases heard by a court | Case					1
Percentage of nationwide greenhouse gas emissions reduction				32.00%
Interim goal for greenhouse gas emissions reduction (fraction)				0.667
Company goal for percentage of carbon dioxide emissions reduction		40.00%
Coal Generation Capacity Expected To Be Retired By 2020 | MW		1,800
Carbon Dioxide Emissions | T			29.2
Climate Change | Electric | Maximum
Air quality
Global temperature increases limit | degreecelsius		2
Climate Change | Electric | Wisconsin
Air quality
Percentage of greenhouse gas emissions reduction by state				41.00%
Climate Change | Electric | Michigan
Air quality
Percentage of greenhouse gas emissions reduction by state				39.00%
Climate Change | Natural gas
Air quality
Carbon Dioxide Emissions | T			26.6
Clean Water Act Cooling Water Intake Structure Rule | Electric
Water quality
Number of compliance options available to meet standard | compliance_option	7
Renewal period for facility permits		5 years
Steam Electric Effluent Limitation Guidelines | Electric
Water quality
Renewal period for facility permits		5 years
Steam Electric Effluent Limitation Guidelines | Electric | Minimum
Water quality
Expected cost to achieve required emissions reductions		$ 70.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured gas plant remediation
Regulatory assets recorded for remediation of manufactured gas plant sites		668.2	$ 676.6
Reserves recorded for remediation of manufactured gas plant sites		$ 617.2	$ 617.2