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	$ 821.2	$ 769.2	$ 1,820.5	$ 1,637.2
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	758.9	707.3	1,542.4	1,436.3
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	698.9	662.3	1,388.3	1,321.3
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	307.6	312.4	619.8	607.4
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	248.7	222.5	491.2	458.5
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	138.0	122.8	267.1	245.7
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.6	4.6	10.2	9.7
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	17.9	18.0	38.7	37.7
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	36.1	22.0	92.6	61.2
Electric | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.2	4.1	19.0	12.5
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1.8	$ 0.9	$ 3.8	$ 3.6