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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,991.1	$ 1,733.0	$ 7,011.9	$ 6,056.3
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,436.9	1,284.6	3,845.5	3,462.8
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,671.8	1,453.6	5,156.7	4,471.8
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,436.9	1,284.6	3,845.5	3,462.8
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,278.4	1,188.0	3,452.0	3,157.4
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	529.7	512.2	1,442.5	1,354.9
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	424.8	396.9	1,173.3	1,074.9
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	316.8	271.9	814.1	705.9
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	7.1	7.0	22.1	21.7
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	39.7	41.1	122.9	119.4
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	102.7	45.0	220.2	145.1
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	3.0	2.7	19.8	21.7
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 13.1	$ 7.8	$ 30.6	$ 19.2