Operating Revenues - Disaggregation of Electric Utility Operating Revenues by Customer Class (Details) - Revenues from contracts with customers - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 7,169.3	$ 7,449.3	$ 7,680.3
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	4,266.1	4,307.7	4,432.4
Wisconsin | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	5,461.7	5,631.8	5,783.0
Wisconsin | Electric | Transferred over time
Disaggregation of Operating Revenues
Revenues from contracts with customers	4,266.1	4,307.7	4,432.4
Wisconsin | Electric | Transferred over time | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	3,920.3	3,894.7	3,987.0
Wisconsin | Electric | Transferred over time | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,743.9	1,608.6	1,636.3
Wisconsin | Electric | Transferred over time | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,325.9	1,384.6	1,408.6
Wisconsin | Electric | Transferred over time | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	821.5	871.9	912.2
Wisconsin | Electric | Transferred over time | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	29.0	29.6	29.9
Wisconsin | Electric | Transferred over time | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	174.0	189.5	210.1
Wisconsin | Electric | Transferred over time | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	130.4	163.1	192.2
Wisconsin | Electric | Transferred over time | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	21.3	23.3	24.1
Wisconsin | Electric | Transferred over time | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 20.1	$ 37.1	$ 19.0