Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred Tax Assets
Retirement Benefit Obligations	$ 40,740	$ 36,551
Net Operating Loss Carryforwards		1,609
Tax Credit Carryforwards	344	1,489
Other, net	1,252	1,589
Total Deferred Tax Assets	42,336	41,238
Deferred Tax Liabilities
Utility Plant Differences	143,800	134,011
Regulatory Assets & Liabilities	6,247	5,239
Other, net	1,307	5,539
Total Deferred Tax Liabilities	151,354	144,789
Net Deferred Tax Liabilities	$ 109,018	$ 103,551