Regulatory Assets and Liabilities - Regulatory Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Regulatory Liabilities [Line Items]
Total regulatory liabilities	$ 203.4	$ 183.9
Total regulatory assets	(8.1)	(8.6)
Cashout and fuel tracker
Regulatory Liabilities [Line Items]
Total regulatory liabilities	29.1	18.3
Provision for other asset retirement
Regulatory Liabilities [Line Items]
Total regulatory liabilities	108.2	103.6
Postretirement benefits other than pension
Regulatory Liabilities [Line Items]
Total regulatory liabilities	66.6	62.8
Unamortized debt issuance costs
Regulatory Liabilities [Line Items]
Total regulatory assets	(0.4)	(0.7)
Unamortized discount on debt
Regulatory Liabilities [Line Items]
Total regulatory assets	$ (0.1)	$ (0.1)