OPERATING REVENUES - DISAGGREGATION OF ELECTRIC UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Utility - Revenues from contracts with customers - Transferred over time - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,515.5	$ 1,406.4	$ 1,409.6
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,176.7	1,136.9	1,114.1
Electric | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,064.7	1,002.0	957.0
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	423.5	420.0	369.4
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	376.2	353.6	355.9
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	256.6	220.0	223.3
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	8.4	8.4	8.4
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	86.7	95.1	105.8
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	11.4	19.6	28.7
Electric | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 13.9	$ 20.2	$ 22.6