PROPERTY, PLANT, AND EQUIPMENT - Plant Retirements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Restructuring Reserve [Roll Forward]
Severance liability at January 1	$ 2.8	$ 3.6
Severance payments	(1.5)	(0.8)
Other	(1.3)	0.0
Severance liability at December 31	0.0	2.8
Property, Plant and Equipment Impairment or Disposal [Abstract]
Regulatory assets	437.9	$ 485.9
Pulliam power plant
Property, Plant and Equipment Impairment or Disposal [Abstract]
Regulatory assets	36.3
Edgewater Unit 4
Property, Plant and Equipment Impairment or Disposal [Abstract]
Regulatory assets	$ 5.3