Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Allowance for loan and lease losses	$ 6,410	$ 4,643
Deferred compensation	1,734	2,110
Actuarial adjustments on retirement benefits	4,592	4,432
State net operating losses	4,730	4,166
Other-than-temporary impairments on equity securities	148	148
Net unrealized holding losses on securities available-for-sale and swaps	2,968	1,316
Other deferred tax assets	1,368	1,243
Gross deferred tax assets	21,950	18,058
Valuation allowance	(3,830)	(3,523)
Total deferred tax assets, net of valuation allowance	18,120	14,535
Deferred tax liabilities:
Mortgage servicing rights	1,463	1,415
Retirement plans	5,263	4,304
Deferred loan fees and costs	1,163	2,614
Acquisition-related fair value adjustments	1,605	1,621
Intangible assets	1,987	1,513
Accounting method change adjustment	1,154	0
Depreciation	937	1,102
Other deferred tax liabilities	963	792
Total deferred tax liabilities	14,535	13,361
Net deferred tax assets	$ 3,585	$ 1,174