OPERATING REVENUES - DISAGGREGATION OF ELECTRIC 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
Electric
Disaggregation of Operating Revenues
Revenues from contracts with customers	1,114.1	1,192.2
Electric | Total retail revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	957.0	1,000.2
Electric | Residential
Disaggregation of Operating Revenues
Revenues from contracts with customers	369.4	381.5
Electric | Small commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	355.9	371.4
Electric | Large commercial and industrial
Disaggregation of Operating Revenues
Revenues from contracts with customers	223.3	238.8
Electric | Other
Disaggregation of Operating Revenues
Revenues from contracts with customers	8.4	8.5
Electric | Wholesale
Disaggregation of Operating Revenues
Revenues from contracts with customers	105.8	142.3
Electric | Resale
Disaggregation of Operating Revenues
Revenues from contracts with customers	28.7	38.5
Electric | Other utility revenues
Disaggregation of Operating Revenues
Revenues from contracts with customers	$ 22.6	$ 11.2