Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Allowance for credit losses, loans and leases	$ 18,113	$ 7,680
Deferred compensation	1,625	1,834
Actuarial adjustments on retirement benefits*	5,408	4,868
State net operating losses	5,621	5,071
Other-than-temporary impairments on equity securities	151	151
Net unrealized holding losses on securities available-for-sale and swaps	483	915
Lease liability	8,227	8,177
Deferred loan fees and costs	583	0
Other deferred tax assets	2,148	1,146
Gross deferred tax assets	42,359	29,842
Valuation allowance	(4,766)	(4,284)
Total deferred tax assets, net of valuation allowance	37,593	25,558
Deferred tax liabilities:
Mortgage servicing rights	1,370	1,404
Retirement plans	5,309	5,299
Deferred loan fees and costs	0	1,330
Acquisition-related fair value adjustments	1,236	1,515
Intangible assets	2,580	1,956
Accounting method change adjustment	385	768
Depreciation	888	1,137
Right of use asset	7,491	7,481
Other deferred tax liabilities	1,820	1,053
Total deferred tax liabilities	21,079	21,943
Net deferred tax assets	$ 16,514	$ 3,615