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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,587.6	$ 1,635.7	$ 5,528.0	$ 5,599.3
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.4	1,218.3	3,269.1	3,370.2
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,336.5	1,385.7	4,212.4	4,292.0
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,185.4	1,218.3	3,269.1	3,370.2
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,085.1	1,110.7	2,958.9	3,033.1
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	454.7	465.3	1,218.1	1,243.3
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	385.6	388.2	1,050.8	1,072.2
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	237.9	249.7	668.0	695.2
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.9	7.5	22.0	22.4
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	53.1	62.0	145.4	175.3
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	29.5	40.7	119.7	139.6
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.4	2.7	16.8	16.9
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 15.3	$ 2.2	$ 28.3	$ 5.3