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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 380.7	$ 367.5	$ 1,122.4	$ 1,050.0
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	42.8	39.5	225.5	185.7
Natural gas | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	37.2	30.1	208.7	163.6
Natural gas | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	22.8	19.6	131.3	107.0
Natural gas | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	14.4	10.5	77.4	56.6
Natural gas | Transportation
Disaggregation of Operating Revenues
Revenues from contracts with customers	3.9	3.8	14.1	14.0
Natural gas | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1.7	$ 5.6	$ 2.7	$ 8.1