PROPERTY, PLANT, AND EQUIPMENT (Details) $ in Millions		12 Months Ended
	Jan. 01, 2017 USD ($) mile	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)
Property, plant, and equipment
Accumulated depreciation		$ 1,633.3		$ 1,595.1
Net property, plant, and equipment		3,823.0		3,662.0
Severance expense			$ 4.6
Pulliam power plant
Property, plant, and equipment
Plant to be retired, net		44.9
Edgewater Unit 4
Property, plant, and equipment
Plant to be retired, net		13.0
UMERC transfer
Property, plant, and equipment
Miles of electric distribution lines transferred | mile	600
Miles of natural gas distribution mains transferred | mile	100
Book value of net assets transferred (including the related deferred income tax liabilities)	$ 20.6
Utility operations
Property, plant, and equipment
Property, plant, and equipment		5,120.8		5,144.5
Accumulated depreciation		1,479.1		1,594.7
Net		3,641.7		3,549.8
CWIP		121.4		111.9
Plant to be retired, net		57.9		0.0
Net property, plant, and equipment		3,821.0		3,661.7
Severance expense		3.6
Non-utility operations
Property, plant, and equipment
Property, plant, and equipment		2.3		0.7
Accumulated depreciation		0.4		0.4
Net		1.9		0.3
CWIP		0.1		0.0
Net property, plant, and equipment		$ 2.0		$ 0.3