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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,543.0	$ 1,563.9	$ 3,629.9	$ 3,940.4
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	999.6	1,021.9	2,034.2	2,083.7
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,207.4	1,249.2	2,700.9	2,875.9
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	999.6	1,021.9	2,034.2	2,083.7
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	920.5	913.1	1,850.9	1,873.8
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	418.7	356.7	823.6	763.4
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	307.1	331.3	630.7	665.2
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	187.8	217.8	382.4	430.1
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.9	7.3	14.2	15.1
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	41.3	44.6	83.4	92.3
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.4	49.4	73.6	90.2
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.1	4.3	12.5	14.4
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 5.3	$ 10.5	$ 13.8	$ 13.0