OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES BY SEGMENT (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Operating Revenues
Operating revenues	$ 1,520.9	$ 1,407.1	$ 1,411.9
Utility
Disaggregation of Operating Revenues
Operating revenues	1,520.9	1,407.1	1,411.9
Utility | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	5.4	0.7	2.3
Utility | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,515.5	1,406.4	1,409.6
Utility | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,176.7	1,136.9	1,114.1
Utility | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 338.8	$ 269.5	$ 295.5