OPERATING REVENUES NATURAL GAS UTILITY OPERATING REVENUS (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Operating Revenues
Operating revenues	$ 350.1	$ 341.6	$ 743.7	$ 731.2
Natural gas
Disaggregation of Operating Revenues
Number of days payment is due			30 days
Utility segment
Disaggregation of Operating Revenues
Operating revenues	350.1		$ 743.7
Revenues from contracts with customers | Utility segment | Transferred over time
Disaggregation of Operating Revenues
Operating revenues	349.4		742.1
Revenues from contracts with customers | Utility segment | Natural gas | Transferred over time
Disaggregation of Operating Revenues
Operating revenues	56.9		167.5
Revenues from contracts with customers | Utility segment | Natural gas | Transferred over time | Residential
Disaggregation of Operating Revenues
Operating revenues	26.5		97.2
Revenues from contracts with customers | Utility segment | Natural gas | Transferred over time | Commercial and industrial
Disaggregation of Operating Revenues
Operating revenues	15.3		59.3
Revenues from contracts with customers | Utility segment | Natural gas | Transferred over time | Total retail
Disaggregation of Operating Revenues
Operating revenues	41.8		156.5
Revenues from contracts with customers | Utility segment | Natural gas | Transferred over time | Transport
Disaggregation of Operating Revenues
Operating revenues	4.0		12.0
Revenues from contracts with customers | Utility segment | Natural gas | Transferred over time | Other utility
Disaggregation of Operating Revenues
Operating revenues	$ 11.1		$ (1.0)