PROPERTY, PLANT, AND EQUIPMENT (Details) $ in Millions	1 Months Ended	6 Months Ended
	Oct. 31, 2017 MW	Jun. 30, 2018 USD ($)
Edgewater Unit 4
Property, plant, and equipment
Plant to be retired, at carrying value		$ 12.3
Plant to be retired, net		14.2
Cost of removal reserve		1.9
Pleasant Prairie power plant
Property, plant, and equipment
Plant to be retired, at carrying value		667.7
Plant to be retired, net		774.2
Cost of removal reserve		106.5
Presque Isle power plant
Property, plant, and equipment
Plant to be retired, at carrying value		189.7
Plant to be retired, net		199.8
Cost of removal reserve		10.1
Pulliam power plant
Property, plant, and equipment
Plant to be retired, at carrying value		42.3
Plant to be retired, net		62.1
Cost of removal reserve		19.8
UMERC
Property, plant, and equipment
Capacity of natural gas-fired generation facility (in megawatts) | MW	180
Wisconsin
Changes to severance liability
Severance liability, balance at beginning of period		29.4
Severance payments		(8.7)
Other		(3.0)
Severance liability, balance at end of period		$ 17.7