OPERATING REVENUES ELECTRIC UTILITY OPERATING REVENUES (Details) - Revenues from contracts with customers - Utility segment - Transferred over time - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 321.0	$ 349.4	$ 715.6	$ 742.1
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	264.6	292.5	541.6	574.6
Electric | Total retail
Disaggregation of Operating Revenues
Revenues from contracts with customers	227.8	246.4	460.5	482.5
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	82.7	90.9	177.7	185.1
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	85.3	88.4	169.4	177.2
Electric | Large commercial and industrial customers
Disaggregation of Operating Revenues
Revenues from contracts with customers	57.7	64.9	109.2	115.9
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	2.1	2.2	4.2	4.3
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	24.2	35.3	53.6	68.9
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	6.9	8.0	16.4	18.1
Electric | Other utility
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 5.7	$ 2.8	$ 11.1	$ 5.1