Allowance for Loan Losses - Changes in the Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	$ 13,134	$ 13,305
Charge-offs	(425)	(131)
Recoveries	105	126
Ending Balance	12,814	13,300
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	1,562	2,018
Charge-offs	(125)	(1)
Recoveries	19	56
Provision	(97)	(504)
Ending Balance	1,359	1,569
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	2,043	2,171
Charge-offs	(193)
Recoveries	9	2
Provision	171	86
Ending Balance	2,030	2,259
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	5,307	4,606
Charge-offs	(44)
Recoveries	14	5
Provision	393	(68)
Ending Balance	5,670	4,543
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	1,910	3,089
Charge-offs	(22)	(89)
Recoveries	58	55
Provision	(196)	(33)
Ending Balance	1,750	3,022
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	834	420
Recoveries		5
Provision	(14)	(12)
Ending Balance	820	413
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	430	442
Recoveries	1
Provision	(20)	(35)
Ending Balance	411	407
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	290	314
Charge-offs	(41)	(41)
Recoveries	4	3
Provision	(6)	78
Ending Balance	247	354
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Beginning balance	758	245
Provision	(231)	488
Ending Balance	$ 527	$ 733