Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets
Retirement Benefit Obligations	$ 38,915	$ 56,804
Net Operating Loss Carryforwards	12,686	23,921
Tax Credit Carryforwards	3,536	3,365
Other, net	1,155	1,426
Total Deferred Tax Assets	56,292	85,516
Deferred Tax Liabilities
Utility Plant Differences	127,932	169,240
Regulatory Assets & Liabilities	9,323	10,594
Other, net	1,894	3,629
Total Deferred Tax Liabilities	139,149	183,463
Net Deferred Tax Liabilities	$ 82,857	$ 97,947