Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Deferred Tax Assets
Retirement Benefit Obligations	$ 11.0	$ 34.1
Net Operating Loss Carryforwards	3.5	4.1
Tax Credit Carryforwards	1.0	0.7
Other, net	1.4	1.3
Total Deferred Tax Assets	16.9	40.2
Deferred Tax Liabilities
Utility Plant Differences	168.3	157.4
Regulatory Assets & Liabilities	11.3	9.4
Other, net	0.7	1.1
Total Deferred Tax Liabilities	180.3	167.9
Net Deferred Tax Liabilities	$ 163.4	$ 127.7