Property, Plant, and Equipment - Plant Retirements (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Plant To Be Retired
Regulatory assets	$ 2,755.2	$ 2,902.3
Deferred tax liabilities	1,368.9	1,365.9
Restructuring Reserve [Roll Forward]
Severance liability at January 1	12.9	25.8
Severance payments	(5.7)	(9.9)
Other	(5.1)	(3.0)
Severance liability at December 31	2.1	$ 12.9
Pleasant Prairie power plant
Plant To Be Retired
Property, plant, and equipment, at carrying value	615.1
Previously deferred unprotected tax benefit	20.6
Regulatory assets	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
Property, plant, and equipment, at carrying value	162.7
Previously deferred unprotected tax benefit	6.4
Regulatory assets	156.3
Deferred tax liabilities	$ 46.5