Income Taxes - Gross Deferred Income Tax Assets and Gross Deferred Income Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred income tax assets:
Allowance for loan losses	$ 8,354	$ 8,058
Deferred compensation	8,910	8,184
Pension and SERP liability	8,770	6,506
Operating lease liabilities	3,567
Unrealized losses on securities transferred to held-to-maturity	643	912
Depreciation	1,060	908
QZAB credit	812
Accrued bonus	708	717
Charitable contributions carryforward	276	389
Nonaccrual interest	115	109
Unrealized (gains) losses on securities available-for-sale	114	(2)
Other	206	181
Gross deferred income tax asset	33,535	25,962
Valuation allowance		(108)
Gross deferred income tax asset, net of valuation allowance	33,535	25,854
Deferred income tax liabilities:
Pension liability	(4,258)	(4,436)
Operating lease right-of-use assets	(3,520)
Deferred origination costs	(516)	(524)
Prepaid expenses	(337)	(228)
Mortgage servicing rights	(338)	(345)
Gross deferred income tax liability	(8,969)	(5,533)
Deferred income tax asset net	$ 24,566	$ 20,321