OPERATING REVENUES ELECTRIC UTILITY OPERATING REVENUES (Details) - Revenues from contracts with customers - Utility segment - Transferred over time - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 956.7	$ 940.4
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	778.8	779.6
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	709.3	673.4
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	302.6	281.4
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	245.1	237.7
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	156.0	148.9
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	5.6	5.4
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.9	28.5
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	31.3	63.7
Electric | Steam
Disaggregation of Operating Revenues
Revenues from contracts with customers	10.1	9.7
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ (0.8)	$ 4.3