OPERATING REVENUES - DISAGGREGATION OF ELECTRIC UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - Utility segment - Transferred over time - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 489.9	$ 477.6
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	320.0	310.7
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	292.5	279.9
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	120.8	117.1
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	104.6	97.3
Electric | Large commercial and industrial customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	64.9	63.3
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.2	2.2
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	22.5	21.8
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.0	5.2
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (1.0)	$ 3.8