Income Taxes - Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax assets:
Allowance for loan losses	$ 10,025	$ 8,354
Deferred compensation	8,833	8,910
Pension and SERP liability	9,911	8,770
Operating lease liabilities	3,917	3,567
Unrealized losses on securities transferred to held-to-maturity	436	643
Depreciation	1,214	1,060
QZAB credit		812
PPP loan origination fees	919
Accrued bonus		708
Charitable contributions carryforward		276
Nonaccrual interest	75	115
Other	305	206
Gross deferred income tax asset	35,635	33,421
Deferred income tax liabilities:
Pension liability	(4,142)	(4,258)
Operating lease right-of-use assets	(3,855)	(3,520)
Deferred origination costs	(584)	(516)
Prepaid expenses	(361)	(337)
Mortgage servicing rights	(217)	(338)
Unrealized (gains) losses on securities available-for-sale	(45)	114
Gross deferred income tax liability	(9,204)	(8,855)
Deferred income tax asset net	$ 26,431	$ 24,566