Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Deferred Tax Assets
Retirement Benefit Obligations	$ 11.2	$ 11.0
Net Operating Loss Carryforwards	0.1	3.5
Tax Credit Carryforwards	1.3	1.0
Other, net	1.3	1.4
Total Deferred Tax Assets	13.9	16.9
Deferred Tax Liabilities
Utility Plant Differences	180.3	168.3
Regulatory Assets & Liabilities	8.9	11.3
Other, net	0.8	0.7
Total Deferred Tax Liabilities	190.0	180.3
Net Deferred Tax Liabilities	$ 176.1	$ 163.4