Regulatory Assets and Liabilities - Plant Retirements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Regulatory assets
Regulatory asset	$ 3,544.1	$ 3,527.6
Deferred tax liabilities	3,721.0	3,609.0
Severance liability for plant retirements
Severance liability at January 1	2.1	15.7	$ 29.4
Severance payments	(0.1)	(7.2)	(10.7)
Other	(1.3)	(6.4)	(3.0)
Severance liability at December 31	0.7	2.1	$ 15.7
Securitization
Regulatory assets
Regulatory asset	105.2	$ 100.0
Edgewater Unit 4
Regulatory assets
Regulatory asset	4.7
Pleasant Prairie power plant
Regulatory assets
Net book value of retired plant	602.7
Deferred unprotected tax benefits	19.6
Regulatory asset	583.1
Deferred tax liabilities	168.7
Pleasant Prairie power plant | Securitization
Regulatory assets
Regulatory asset	100.0
Presque Isle power plant
Regulatory assets
Net book value of retired plant	161.0
Deferred unprotected tax benefits	6.0
Regulatory asset	155.0
Deferred tax liabilities	46.0
Pulliam power plant
Regulatory assets
Regulatory asset	$ 42.6