COMMITMENTS AND CONTINGENCIES - ENVIRONMENTAL MATTERS (Details) T in Millions, $ in Millions	1 Months Ended			12 Months Ended	15 Months Ended
	Jun. 30, 2021 area	May 31, 2021	Dec. 31, 2020 USD ($) micrograms performance_obligations	Dec. 31, 2021 USD ($) T MW	Mar. 31, 2019 MW	Oct. 31, 2021 performance_obligations	Feb. 28, 2021 compliance_option
Manufactured Gas Plant Remediation
Regulatory assets			$ 449.4	$ 347.9
Environmental remediation costs
Manufactured Gas Plant Remediation
Regulatory assets			$ 116.1	$ 120.1
National Ambient Air Quality Standards | Electric
Air Quality
Number of changes to the 2015 ozone standards | performance_obligations						0
Number of counties to have boundaries revised | area	13
Number of nonattainment areas designated by the EPA | area	6
Number of revisions necessary to meet the 2012 standard for particulate matter | performance_obligations			0
Current level of micrograms per cubic meter that particulate matter needs to be below | micrograms			12
National Ambient Air Quality Standards | Electric | Minimum
Air Quality
New possible number of micrograms per cubic meter that particulate matter may be lowered to | micrograms			10
National Ambient Air Quality Standards | Electric | Maximum
Air Quality
New possible number of micrograms per cubic meter that particulate matter may be lowered to | micrograms			11
Climate Change | Electric
Air Quality
Number of rules that regulate GHG emissions from electric generating units | compliance_option							0
Capacity of coal generation retired since the beginning of 2018, in megawatts | MW					300
Capacity of fossil-fueled generation to be retired by 2025, in megawatts | MW				1,600
Company near-term goal for percentage of carbon dioxide emission reduction goal by 2025		60.00%
Company goal for percentage of carbon dioxide emissions reduction below 2005 levels by 2030		80.00%
Carbon dioxide emissions | T				6.0
Climate Change | Natural gas
Air Quality
Carbon dioxide emissions | T				3.5
Steam Electric Effluent Guidelines | Electric
Water Quality
Expected capital investment to achieve required discharge limits under ELG rule				$ 10.0
Manufactured Gas Plant Remediation | Natural gas
Manufactured Gas Plant Remediation
Reserves recorded for future environmental remediation of manufactured gas plant sites			$ 88.3	95.0
Manufactured Gas Plant Remediation | Natural gas | Environmental remediation costs
Manufactured Gas Plant Remediation
Regulatory assets			$ 116.1	$ 120.1
Renewables, Efficiency, and Conservation | Wisconsin | Electric
Renewables, Efficiency, and Conservation
Annual renewable portfolio requirement for Wisconsin, as a percent				10.00%
Required renewable energy percent achieved				9.74%
Percent of annual operating revenues used to fund renewable program				1.20%