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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 338.0	$ 314.3	$ 741.7	$ 682.5
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	287.6	265.0	559.0	536.3
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	262.0	232.3	505.3	470.0
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	102.8	98.4	204.7	199.5
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	93.4	80.7	177.5	164.0
Electric | Large commercial and industrial customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	63.7	51.1	118.9	102.3
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.1	2.1	4.2	4.2
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.7	23.1	39.6	45.5
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	0.9	5.3	6.8	10.6
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 4.0	$ 4.3	$ 7.3	$ 10.2