Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Allowance for loan and lease losses	$ 4,643	$ 5,984
Deferred compensation	2,110	2,541
Actuarial adjustments on retirement benefits	4,432	7,714
State net operating losses	4,166	2,725
Other-than-temporary impairments on equity securities	148	331
Net unrealized holding losses on securities available-for-sale and swaps	1,316	2,762
Other deferred tax assets	1,243	5,712
Gross deferred tax assets	18,058	27,769
Valuation allowance	(3,523)	(2,341)
Total deferred tax assets, net of valuation allowance	14,535	25,428
Deferred tax liabilities:
Mortgage servicing rights	1,415	2,302
Retirement plans	4,304	6,265
Deferred loan fees and costs	2,614	615
Acquisition-related fair value adjustments	1,621	2,097
Intangible assets	1,513	1,491
Depreciation	1,102	1,401
Other deferred tax liabilities	792	1,692
Total deferred tax liabilities	13,361	15,863
Net deferred tax assets	$ 1,174	$ 9,565