OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Revenues from contracts with customers - Utility segment - Transferred over time - 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
Revenues from contracts with customers	$ 821.2	$ 769.2	$ 1,820.5	$ 1,637.2
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	62.3	61.9	278.1	200.9
Natural gas | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	101.9	52.2	267.2	196.0
Natural gas | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	70.3	39.2	183.0	138.7
Natural gas | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.6	13.0	84.2	57.3
Natural gas | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	3.8	3.5	9.1	8.5
Natural gas | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (43.4)	$ 6.2	$ 1.8	$ (3.6)