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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,810.0	$ 2,121.2	$ 4,668.2	$ 5,020.8
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,178.5	1,221.1	2,382.3	2,408.6
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,418.4	1,550.6	3,406.6	3,484.9
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,178.5	1,221.1	2,382.3	2,408.6
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,107.7	1,103.4	2,225.6	2,173.6
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	459.1	449.7	945.6	912.8
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	401.5	378.4	795.1	748.5
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	239.9	268.1	469.7	497.3
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.2	7.2	15.2	15.0
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	30.4	40.8	64.6	83.2
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.9	60.7	72.5	117.5
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.6	4.7	15.6	16.8
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 3.9	$ 11.5	$ 4.0	$ 17.5