OPERATING REVENUES - DISAGGREGATION OF OPERATING REVENUES FOR UTILITY SEGMENT (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Operating Revenues
Total operating revenues	$ 827.5	$ 769.5	$ 1,832.7	$ 1,640.5
Utility segment
Disaggregation of Operating Revenues
Total operating revenues	827.5	769.5	1,832.7	1,640.5
Utility segment | Other operating revenues
Disaggregation of Operating Revenues
Other operating revenues	6.3	0.3	12.2	3.3
Utility segment | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	821.2	769.2	1,820.5	1,637.2
Utility segment | Electric | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	758.9	707.3	1,542.4	1,436.3
Utility segment | Natural gas | Transferred over time | Revenues from contracts with customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 62.3	$ 61.9	$ 278.1	$ 200.9