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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 380.7	$ 367.5	$ 1,122.4	$ 1,050.0
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	337.9	328.0	896.9	864.3
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	306.9	290.8	812.2	760.8
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	117.2	118.5	321.9	318.0
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	109.4	104.1	286.9	268.1
Electric | Large commercial and industrial customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	78.2	66.1	197.1	168.4
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.1	2.1	6.3	6.3
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	24.8	27.3	64.4	72.8
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	1.3	4.3	8.1	14.9
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 4.9	$ 5.6	$ 12.2	$ 15.8