OPERATING REVENUES - DISAGGREGATION OF ELECTRIC UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,733.0	$ 1,638.6	$ 6,056.3	$ 5,268.5
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,284.6	1,212.9	3,462.8	3,247.1
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,453.6	1,363.4	4,471.8	4,064.3
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,284.6	1,212.9	3,462.8	3,247.1
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,188.0	1,125.3	3,157.4	2,976.2
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	512.2	500.8	1,354.9	1,324.4
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	396.9	373.6	1,074.9	1,004.3
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	271.9	244.0	705.9	626.4
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.0	6.9	21.7	21.1
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	41.1	46.5	119.4	129.9
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	45.0	33.7	145.1	107.3
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.7	2.5	21.7	15.0
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 7.8	$ 4.9	$ 19.2	$ 18.7