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	$ 955.2	$ 897.9	$ 2,775.7	$ 2,535.1
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	905.0	853.2	2,447.4	2,289.5
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	840.4	803.0	2,228.7	2,124.3
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	385.3	373.8	1,005.1	981.2
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	282.4	265.3	773.6	723.8
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	168.0	159.4	435.1	405.1
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.7	4.5	14.9	14.2
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	16.3	19.2	55.0	56.9
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.8	29.1	135.4	90.3
Electric | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.7	2.5	21.7	15.0
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 2.8	$ (0.6)	$ 6.6	$ 3.0