PROPERTY, PLANT, AND EQUIPMENT (Details) $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2017 MW	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)
Edgewater Unit 4
Property, plant, and equipment
Plant to be retired, net			$ 12.7
Pleasant Prairie power plant
Property, plant, and equipment
Plant to be retired, net			674.1
Presque Isle power plant
Property, plant, and equipment
Plant to be retired, net			188.7
Pulliam power plant
Property, plant, and equipment
Plant to be retired, net			$ 43.6
UMERC
Property, plant, and equipment
Capacity of natural gas-fired generation facility (in megawatts) | MW	180
Wisconsin
Property, plant, and equipment
Severance expense		$ 29.4