COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions	1 Months Ended			12 Months Ended
	Oct. 31, 2014 compliance_option	Jun. 30, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2017 USD ($) T MW	Dec. 31, 2016 USD ($) T	Dec. 31, 2015	Sep. 30, 2016 Case	Jun. 01, 2015 USD ($)
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%
Capacity of coal generation expected to be retired by 2020 | MW				308
Carbon dioxide emissions | T				5.7	5.2
Climate Change | Natural gas
Air Quality
Carbon dioxide emissions | T				3.5	3.4
Climate Change | Wisconsin | Electric
Air Quality
Percentage of greenhouse gas emissions reduction by state						41.00%
Clean Water Act Cooling Water Intake Structure Rule | Electric
Water Quality
Number of compliance options available to meet standard | compliance_option	7
Steam Electric Effluent Guidelines | Electric
Water Quality
Renewal period for facility permits				5 years
Expected cost to achieve required emissions reduction				$ 20.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured Gas Plant Remediation
Regulatory assets recorded for remediation of manufactured gas plant sites				116.0	$ 116.0
Reserves recorded for remediation of manufactured gas plant sites				$ 99.6	$ 97.2
Renewables, Efficiency, and Conservation | Wisconsin | Electric
Renewables, Efficiency, and Conservation
Renewable portfolio requirement for Wisconsin, as a percent				10.00%
Renewable energy percent				9.74%
Percent of annual operating revenues used to fund renewable program				1.20%
Consent Decree - Weston and Pulliam Power Plants | Electric
Consent Decrees
Beneficial environmental projects amount			$ 6.0
Civil penalty			$ 1.2
Regulatory asset for undepreciated book value of retired plants								$ 11.5
Joint Ownership Power Plants Consent Decree - Columbia and Edgewater | Electric
Consent Decrees
Beneficial environmental projects amount		$ 1.3
Civil penalty		$ 0.4