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PROPERTY, PLANT, AND EQUIPMENT (Details) $ in Millions	1 Months Ended	9 Months Ended
PROPERTY, PLANT, AND EQUIPMENT (Details) $ in Millions	Oct. 31, 2017 MW	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Property, plant, and equipment
Cost of removal reserve classified as a regulatory liability		$ 3,985.9	$ 3,760.4
Net book value of plant classified as a regulatory asset		3,673.8	$ 2,840.4
Edgewater Unit 4
Property, plant, and equipment
Plant to be retired, at carrying value		8.3
Cost of removal reserve classified as a regulatory liability		1.9
Net book value of plant classified as a regulatory asset		10.2
Pleasant Prairie power plant
Property, plant, and equipment
Plant to be retired, at carrying value		653.3
Cost of removal reserve classified as a regulatory liability		102.5
Net book value of plant classified as a regulatory asset		755.8
Presque Isle power plant
Property, plant, and equipment
Plant to be retired, at carrying value		186.9
Cost of removal reserve classified as a regulatory liability		10.5
Plant to be retired, net		197.4
Pulliam power plant
Property, plant, and equipment
Plant to be retired, at carrying value		40.8
Cost of removal reserve classified as a regulatory liability		20.3
Plant to be retired, net		61.1
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		(9.5)
Other		(3.0)
Severance liability, balance at end of period		$ 16.9