OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES FOR UTILITY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Operating Revenues
Total operating revenues	$ 491.5	$ 478.9
Utility segment
Disaggregation of Operating Revenues
Total operating revenues	491.5	478.9
Utility segment | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	1.6	1.3
Utility segment | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	489.9	477.6
Utility segment | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	320.0	310.7
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 169.9	$ 166.9