Income Taxes - Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred income tax assets:
Allowance for loan losses	$ 10,419	$ 9,852
AMT credit	10,234	7,041
Deferred compensation	9,684	8,495
Pension and SERP liability	7,658	8,714
Unrealized losses on securities transferred to held-to-maturity	3,161	4,667
Depreciation	968	673
Accrued bonus	612	508
Unrealized (gains) losses on securities available-for-sale	357	108
Charitable contributions carryforward	266
Acquisition premium	128	231
Nonaccrual interest	125	138
Limited partnerships	30	52
Investments write down	26	26
Other	220	173
Gross deferred income tax asset	43,888	40,678
Valuation allowance	(108)
Gross deferred income tax asset,net of valuation allowance	43,780	40,678
Deferred income tax liabilities:
Mortgage servicing rights	(651)	(521)
Gross deferred income tax liability	(651)	(521)
Deferred income tax asset net	$ 43,129	$ 40,157