OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES FOR UTILITY SEGMENT (Details) - 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
Total operating revenues	$ 958.8	$ 900.2	$ 2,791.5	$ 2,540.7
Utility segment
Disaggregation of Operating Revenues
Total operating revenues	958.8	900.2	2,791.5	2,540.7
Utility segment | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	3.6	2.3	15.8	5.6
Utility segment | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	955.2	897.9	2,775.7	2,535.1
Utility segment | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	905.0	853.2	2,447.4	2,289.5
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 50.2	$ 44.7	$ 328.3	$ 245.6