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, 2021	Mar. 31, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 999.3	$ 868.0
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	783.5	729.0
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	689.4	659.0
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	312.2	295.0
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	242.5	236.0
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	129.1	122.9
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	5.6	5.1
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	20.8	19.7
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	56.5	39.2
Electric | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.8	8.4
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2.0	$ 2.7