OPERATING REVENUES - DISAGGREGATION OF NATURAL GAS UTILITY OPERATING REVENUES BY CUSTOMER CLASS (Details) - Utility - Revenues from contracts with customers - Transferred over time - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 1,409.6	$ 1,497.7
Natural gas
Disaggregation of Operating Revenues
Revenues from contracts with customers	295.5	305.5
Natural gas | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	284.3	285.3
Natural gas | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	178.4	177.7
Natural gas | Commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	105.9	107.6
Natural gas | Transport
Disaggregation of Operating Revenues
Revenues from contracts with customers	17.1	19.7
Natural gas | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (5.9)	$ 0.5