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	$ 381.1	$ 367.7	$ 1,127.6	$ 1,049.9
Utility segment
Disaggregation of Operating Revenues
Total operating revenues	381.1	367.7	1,127.6	1,049.9
Utility segment | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	0.4	0.2	5.2	(0.1)
Utility segment | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	380.7	367.5	1,122.4	1,050.0
Utility segment | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	337.9	328.0	896.9	864.3
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 42.8	$ 39.5	$ 225.5	$ 185.7