Allowance for Loan Losses - Ending Allocation of Allowance for Loan Losses and Loan Balances Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	$ 439	$ 188
Collectively evaluated for impairment	12,699	12,942
Total	13,300	13,305	$ 14,433	$ 14,361
Individually evaluated for impairment	6,226	6,539
Collectively evaluated for impairment	1,068,767	1,048,711
Total	1,075,240	1,055,506
Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	162	175
Loan balance of loans acquired with credit deterioration	247	256
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	243	82
Collectively evaluated for impairment	1,245	1,850
Total	1,569	2,018	1,446	1,478
Individually evaluated for impairment	1,722	1,983
Collectively evaluated for impairment	137,640	133,391
Total	139,449	135,462
Commercial and Agriculture [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	81	86
Loan balance of loans acquired with credit deterioration	87	88
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	81	4
Collectively evaluated for impairment	2,178	2,167
Total	2,259	2,171	2,372	2,467
Individually evaluated for impairment	1,875	1,896
Collectively evaluated for impairment	166,029	159,468
Total	167,904	161,364
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	4,543	4,606
Total	4,543	4,606	4,711	4,657
Individually evaluated for impairment	358	359
Collectively evaluated for impairment	400,053	395,572
Total	400,411	395,931
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for impairment	115	102
Collectively evaluated for impairment	2,826	2,898
Total	3,022	3,089	4,114	4,086
Individually evaluated for impairment	1,657	1,686
Collectively evaluated for impairment	253,761	245,454
Total	255,578	247,308
Residential Real Estate [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	81	89
Loan balance of loans acquired with credit deterioration	160	168
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	413	420
Total	413	420	408	371
Collectively evaluated for impairment	55,266	56,293
Total	55,266	56,293
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	407	442
Total	407	442	496	538
Individually evaluated for impairment	614	614
Collectively evaluated for impairment	37,421	40,556
Total	38,035	41,170
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	354	314
Total	354	314	358	382
Individually evaluated for impairment		1
Collectively evaluated for impairment	18,597	17,977
Total	18,597	17,978
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collectively evaluated for impairment	733	245
Total	$ 733	$ 245	$ 528	$ 382