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	$ 338.0	$ 314.3	$ 741.7	$ 682.5
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	50.4	49.3	182.7	146.2
Natural gas | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	65.5	42.0	171.5	133.5
Natural gas | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	41.2	29.0	108.5	87.4
Natural gas | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	24.3	13.0	63.0	46.1
Natural gas | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	4.4	4.2	10.2	10.2
Natural gas | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (19.5)	$ 3.1	$ 1.0	$ 2.5