Allowance for Loan Losses - Changes in the Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 13,047	$ 14,547	$ 13,305	$ 14,361
Charge-offs	(309)	(1,183)	(797)	(1,539)
Recoveries	208	87	438	1,929
Provision				(1,300)
Ending Balance	12,946	13,451	12,946	13,451
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	1,608	1,557	2,018	1,478
Charge-offs	(10)	(828)	(11)	(870)
Recoveries	51	44	134	79
Provision	(38)	937	(530)	1,023
Ending Balance	1,611	1,710	1,611	1,710
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	2,010	2,393	2,171	2,467
Charge-offs	(91)	(124)	(301)	(166)
Recoveries	8	1	26	53
Provision	146	40	177	(44)
Ending Balance	2,073	2,310	2,073	2,310
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	4,739	4,969	4,606	4,657
Charge-offs	(38)	(23)	(38)	(23)
Recoveries	33	6	42	1,365
Provision	539	(409)	663	(1,456)
Ending Balance	5,273	4,543	5,273	4,543
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	2,676	3,899	3,089	4,086
Charge-offs	(116)	(55)	(312)	(280)
Recoveries	77	23	164	384
Provision	(158)	(412)	(462)	(735)
Ending Balance	2,479	3,455	2,479	3,455
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	482	366	420	371
Charge-offs		(115)		(115)
Recoveries	13	6	32	8
Provision	172	167	215	160
Ending Balance	667	424	667	424
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	425	476	442	538
Recoveries	2		2
Provision	(3)	(46)	(20)	(108)
Ending Balance	424	430	424	430
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	324	358	314	382
Charge-offs	(54)	(38)	(135)	(85)
Recoveries	24	7	38	40
Provision	31	10	108	0
Ending Balance	325	337	325	337
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	783	529	245	382
Provision	(689)	(287)	(151)	(140)
Ending Balance	$ 94	$ 242	$ 94	$ 242