Regulatory Assets and Liabilities - Plant Retirements (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Regulatory assets
Regulatory asset	$ 3,367.1	$ 3,544.1
Deferred tax liabilities	3,909.0	3,721.0
Securitization
Regulatory assets
Regulatory asset	100.7	$ 105.2
Edgewater Unit 4
Regulatory assets
Regulatory asset	3.6
Pleasant Prairie power plant
Regulatory assets
Net book value of retired plant	585.7
Deferred unprotected tax benefits	18.5
Regulatory asset	567.2
Deferred tax liabilities	164.1
Pleasant Prairie power plant | Securitization
Regulatory assets
Regulatory asset	100.0
Presque Isle power plant
Regulatory assets
Net book value of retired plant	163.3
Deferred unprotected tax benefits	5.6
Regulatory asset	157.7
Deferred tax liabilities	46.7
Pulliam power plant
Regulatory assets
Regulatory asset	$ 38.0