Property, Plant, and Equipment - Plant Retirements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Plant To Be Retired
Net book value of plant classified as a regulatory asset	$ 3,527.6	$ 3,855.8
Deferred tax liabilities	3,609.0	3,436.9
Pleasant Prairie power plant
Plant To Be Retired
Net book value of plant, representing book value less cost of removal and accumulated depreciation	615.1
Previously deferred unprotected tax benefit	20.6
Net book value of plant classified as a regulatory asset	594.5
Deferred tax liabilities	172.1
Pleasant Prairie power plant's book value to be securitized	100.0
Presque Isle power plant
Plant To Be Retired
Net book value of plant, representing book value less cost of removal and accumulated depreciation	162.7
Previously deferred unprotected tax benefit	6.4
Net book value of plant classified as a regulatory asset	156.3
Deferred tax liabilities	46.5
Pulliam power plant
Plant To Be Retired
Net book value of plant classified as a regulatory asset	36.3
Wisconsin
Restructuring Reserve [Roll Forward]
Severance liability, balance at beginning of period	15.7	29.4
Severance payments	(7.2)	(10.7)
Other	(6.4)	(3.0)
Severance liability, balance at end of period	2.1	$ 15.7
Edgewater Unit 4
Plant To Be Retired
Net book value of plant classified as a regulatory asset	$ 5.3