Regulatory Assets and Liabilities - Plant Retirements (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	May 31, 2021
Regulatory assets
Total regulatory assets	$ 3,274.7	$ 3,306.9
Deferred tax liabilities	4,198.0	4,072.5
Securitization
Regulatory assets
Total regulatory assets	85.9	$ 92.4
Edgewater Unit 4
Regulatory assets
Total regulatory assets	2.1
Pleasant Prairie power plant
Regulatory assets
Net book value of retired plant	542.4
Deferred unprotected tax benefits	16.4
Total regulatory assets	526.0
Deferred tax liabilities	147.8
Pleasant Prairie power plant | Securitization
Regulatory assets
Total regulatory assets			$ 100.0
Presque Isle power plant
Regulatory assets
Net book value of retired plant	152.9
Deferred unprotected tax benefits	4.8
Total regulatory assets	148.1
Deferred tax liabilities	41.5
Pulliam power plant
Regulatory assets
Total regulatory assets	$ 33.0