Property, Plant, and Equipment - Plant Retirements (Details) $ in Millions	1 Months Ended		12 Months Ended
	Oct. 31, 2017 MW	Aug. 31, 2016 MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Plant To Be Retired
Accumulated depreciation			$ 8,515.9	$ 8,618.5
Net property, plant, and equipment			22,000.9	21,347.0
Regulatory Assets			3,855.8	2,840.4
Regulatory Liabilities			4,288.4	$ 3,760.4
UMERC
Plant To Be Retired
Capacity of natural gas-fired generation facility (in megawatts) | MW	180	180
Pleasant Prairie power plant
Plant To Be Retired
Property, plant, and equipment, at carrying value			645.9
Regulatory Assets			749.5
Regulatory Liabilities			103.6
Presque Isle power plant
Plant To Be Retired
Property, plant, and equipment to be retired, net			185.4
Property, plant, and equipment, at carrying value			174.8
Regulatory Liabilities			10.6
Pulliam power plant
Plant To Be Retired
Property, plant, and equipment, at carrying value			33.8
Regulatory Assets			57.2
Regulatory Liabilities			23.4
Wisconsin
Restructuring Reserve [Roll Forward]
Restructuring Reserve Beginning of Period			29.4
Payments for Restructuring			(10.7)
Restructuring Reserve, Accrual Adjustment			(3.0)
Restructuring Reserve End of Period			15.7
Edgewater Unit 4
Plant To Be Retired
Property, plant, and equipment, at carrying value			8.1
Regulatory Assets			10.0
Regulatory Liabilities			$ 1.9