Loans - Contractual Aging of Recorded Investment in Past Due Loans by Portfolio Classification of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 2,375	$ 3,727
Current	1,339,304	1,299,263
Loans after net deferred loan fees and costs	1,341,679	1,302,990	$ 1,247,641
Recorded Investment > 90 Days and Accruing		0	0
30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	290	322
60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1	941
Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,084	2,464
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,303	1,492
Current	174,178	173,824
Loans after net deferred loan fees and costs	175,481	175,316	173,976
Consumer Real Estate [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	284	269
Consumer Real Estate [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due		191
Consumer Real Estate [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,019	1,032
Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	88	88
Current	179,551	188,738
Loans after net deferred loan fees and costs	179,639	188,826	194,130
Agricultural Real Estate [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	88	88
Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due		176
Current	100,147	94,314
Loans after net deferred loan fees and costs	100,147	94,490	109,707
Agricultural [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due		176
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	183	185
Current	617,329	587,469
Loans after net deferred loan fees and costs	617,512	587,654	568,991
Commercial Real Estate [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	183	185
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	794	1,733
Current	213,547	202,310
Loans after net deferred loan fees and costs	214,341	204,043	151,647
Commercial and Industrial [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due		750
Commercial and Industrial [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	794	983
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	7	53
Current	54,552	52,608
Loans after net deferred loan fees and costs	54,559	52,661	$ 49,190
Consumer [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	6	$ 53
Consumer [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 1