OPERATING REVENUES - DISAGGREGATION OF ELECTRIC UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,654.3	$ 1,543.0	$ 4,323.3	$ 3,629.9
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,083.2	999.6	2,178.2	2,034.2
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,295.9	1,207.4	3,018.2	2,700.9
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,083.2	999.6	2,178.2	2,034.2
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	997.0	920.5	1,969.4	1,850.9
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	419.0	418.7	842.7	823.6
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	346.6	307.1	678.0	630.7
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	224.5	187.8	434.0	382.4
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.9	6.9	14.7	14.2
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	38.6	41.3	78.3	83.4
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	37.4	28.4	100.1	73.6
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.2	4.1	19.0	12.5
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 6.0	$ 5.3	$ 11.4	$ 13.8