Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred Tax Assets
Retirement Benefit Obligations	$ 34.1	$ 40.7
Net Operating Loss Carryforwards	4.1
Tax Credit Carryforwards	0.7	0.3
Other, net	1.3	1.3
Total Deferred Tax Assets	40.2	42.3
Deferred Tax Liabilities
Utility Plant Differences	157.4	143.8
Regulatory Assets & Liabilities	9.4	6.2
Other, net	1.1	1.3
Total Deferred Tax Liabilities	167.9	151.3
Net Deferred Tax Liabilities	$ 127.7	$ 109.0