Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets
Retirement Benefit Obligations	$ 56,804	$ 43,543
Net Operating Loss Carryforwards	23,921	10,500
Tax Credit Carryforwards	3,365	2,677
Other, net	1,426	4,019
Total Deferred Tax Assets	85,516	60,739
Deferred Tax Liabilities
Utility Plant Differences	169,240	142,412
Regulatory Assets & Liabilities	10,594	5,445
Other, net	3,629	348
Total Deferred Tax Liabilities	183,463	148,205
Net Deferred Tax Liabilities	$ 97,947	$ 87,466