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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,563.9	$ 1,673.6	$ 3,940.4	$ 3,963.6
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,021.9	1,084.2	2,083.7	2,151.9
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,249.2	1,320.6	2,875.9	2,906.3
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,021.9	1,084.2	2,083.7	2,151.9
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	913.1	995.8	1,873.8	1,922.4
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	356.7	393.7	763.4	778.0
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	331.3	353.3	665.2	684.0
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	217.8	241.6	430.1	445.5
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.3	7.2	15.1	14.9
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	44.6	58.4	92.3	113.3
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	49.4	25.1	90.2	98.9
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.3	4.5	14.4	14.2
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 10.5	$ 0.4	$ 13.0	$ 3.1