OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES FOR UTILITY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Operating Revenues
Total operating revenues	$ 337.0	$ 381.0	$ 764.2	$ 872.5
Utility segment
Disaggregation of Operating Revenues
Total operating revenues	337.0	381.0	764.2	872.5
Utility segment | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	(1.1)	(0.3)	0.1	1.3
Utility segment | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	338.1	381.3	764.1	871.2
Utility segment | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	286.8	319.8	587.1	639.8
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 51.3	$ 61.5	$ 177.0	$ 231.4