Allowance for Loan Losses - Changes in the Allowance for Loan Losses and Loan Balances Outstanding (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 14,547	$ 14,707	$ 14,361	$ 14,361	$ 14,268
Charge-offs	(1,183)	(634)	(1,539)		(1,524)
Recoveries	87	287	1,929		816
Provision		400	(1,300)	(1,300)	1,200
Ending Balance	13,451	14,760	13,451	13,451	14,760
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	1,557	1,395	1,478	1,478	1,819
Charge-offs	(828)	(187)	(870)		(187)
Recoveries	44	127	79		158
Provision	937	43	1,023		(412)
Ending Balance	1,710	1,378	1,710	1,710	1,378
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	4,969	3,153	4,657	4,657	4,334
Charge-offs	(23)	(17)	(23)		(81)
Recoveries	6	15	1,365		105
Provision	(409)	1,788	(1,456)		581
Ending Balance	4,543	4,939	4,543	4,543	4,939
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	3,899	3,470	4,086	4,086	3,747
Charge-offs	(55)	(238)	(280)		(779)
Recoveries	23	124	384		289
Provision	(412)	866	(735)		965
Ending Balance	3,455	4,222	3,455	3,455	4,222
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	366	434	371	371	428
Charge-offs	(115)		(115)
Recoveries	6	1	8		4
Provision	167	(7)	160		(4)
Ending Balance	424	428	424	424	428
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	476	584	538	538	822
Recoveries		9			60
Provision	(46)	65	(108)		(224)
Ending Balance	430	658	430	430	658
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	358	190	382	382	200
Charge-offs	(38)	(28)	(85)		(115)
Recoveries	7	8	40		38
Provision	10	204			251
Ending Balance	337	374	337	337	374
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	2,393	5,073	2,467	2,467	2,221
Charge-offs	(124)	(164)	(166)		(362)
Recoveries	1	3	53		162
Provision	40	(2,230)	(44)		661
Ending Balance	2,310	2,682	2,310	2,310	2,682
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	529	408	382	382	697
Provision	(287)	(329)	(140)		(618)
Ending Balance	$ 242	$ 79	$ 242	$ 242	$ 79