Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Retirement Benefit Obligations	$ 36,551	$ 32,249
Net Operating Loss Carryforwards	1,609	10,773
Tax Credit Carryforwards	1,489	2,704
Other, net	1,589	1,571
Total Deferred Tax Assets	41,238	47,297
Deferred Tax Liabilities
Utility Plant Differences	134,011	132,682
Regulatory Assets & Liabilities	5,239	6,429
Other, net	5,539	5,964
Total Deferred Tax Liabilities	144,789	145,075
Net Deferred Tax Liabilities	$ 103,551	$ 97,778