Allowance for Loan Losses - Changes in the Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 13,300	$ 14,433	$ 13,305	$ 14,361
Charge-offs	(357)	(230)	(488)	(356)
Recoveries	104	1,644	230	1,842
Provision		(1,300)		(1,300)
Ending Balance	13,047	14,547	13,047	14,547
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	1,569	1,446	2,018	1,478
Charge-offs		(20)	(1)	(42)
Recoveries	27	30	83	35
Provision	12	101	(492)	86
Ending Balance	1,608	1,557	1,608	1,557
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	4,543	4,711	4,606	4,657
Recoveries	4	1,319	9	1,359
Provision	192	(1,061)	124	(1,047)
Ending Balance	4,739	4,969	4,739	4,969
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	3,022	4,114	3,089	4,086
Charge-offs	(107)	(129)	(196)	(225)
Recoveries	32	272	87	361
Provision	(271)	(358)	(304)	(323)
Ending Balance	2,676	3,899	2,676	3,899
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	413	408	420	371
Recoveries	14	1	19	2
Provision	55	(43)	43	(7)
Ending Balance	482	366	482	366
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	407	496	442	538
Provision	18	(20)	(17)	(62)
Ending Balance	425	476	425	476
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	354	358	314	382
Charge-offs	(40)	(39)	(81)	(47)
Recoveries	11	19	14	33
Provision	(1)	20	77	(10)
Ending Balance	324	358	324	358
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	2,259	2,372	2,171	2,467
Charge-offs	(210)	(42)	(210)	(42)
Recoveries	16	3	18	52
Provision	(55)	60	31	(84)
Ending Balance	2,010	2,393	2,010	2,393
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	733	528	245	382
Provision	50	1	538	147
Ending Balance	$ 783	$ 529	$ 783	$ 529