Allowance for Loan Losses - Ending Allocation of Allowance for Loan Losses and Loan Balances Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	$ 179	$ 125
Allowance for loan losses, Collectively evaluated for impairment	12,598	12,883
Allowance for loan losses, Total	12,814	13,134	$ 13,300	$ 13,305
Outstanding loan balances, Individually evaluated for impairment	3,728	3,460
Outstanding loan balances, Collectively evaluated for impairment	1,149,831	1,160,986
Outstanding loan balances, Total	1,153,758	1,164,661
Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	37	126
Loan balance of loans acquired with credit deterioration	199	215
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	60	4
Allowance for loan losses, Collectively evaluated for impairment	1,299	1,476
Allowance for loan losses, Total	1,359	1,562	1,569	2,018
Outstanding loan balances, Individually evaluated for impairment	1,025	438
Outstanding loan balances, Collectively evaluated for impairment	135,971	151,948
Outstanding loan balances, Total	137,076	152,473
Commercial and Agriculture [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration		82
Loan balance of loans acquired with credit deterioration	80	87
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	8	6
Allowance for loan losses, Collectively evaluated for impairment	2,022	2,037
Allowance for loan losses, Total	2,030	2,043	2,259	2,171
Outstanding loan balances, Individually evaluated for impairment	532	1,010
Outstanding loan balances, Collectively evaluated for impairment	162,453	163,089
Outstanding loan balances, Total	162,985	164,099
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	5,670	5,307
Allowance for loan losses, Total	5,670	5,307	4,543	4,606
Outstanding loan balances, Individually evaluated for impairment	43	44
Outstanding loan balances, Collectively evaluated for impairment	436,117	425,579
Outstanding loan balances, Total	436,160	425,623
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	105	109
Allowance for loan losses, Collectively evaluated for impairment	1,608	1,757
Allowance for loan losses, Total	1,750	1,910	3,022	3,089
Outstanding loan balances, Individually evaluated for impairment	1,335	1,360
Outstanding loan balances, Collectively evaluated for impairment	265,976	267,247
Outstanding loan balances, Total	267,430	268,735
Residential Real Estate [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration	37	44
Loan balance of loans acquired with credit deterioration	119	128
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	820	834
Allowance for loan losses, Total	820	834	413	420
Outstanding loan balances, Collectively evaluated for impairment	95,856	97,531
Outstanding loan balances, Total	95,856	97,531
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	6	6
Allowance for loan losses, Collectively evaluated for impairment	405	424
Allowance for loan losses, Total	411	430	407	442
Outstanding loan balances, Individually evaluated for impairment	793	608
Outstanding loan balances, Collectively evaluated for impairment	37,135	38,853
Outstanding loan balances, Total	37,928	39,461
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	247	290
Allowance for loan losses, Total	247	290	354	314
Outstanding loan balances, Collectively evaluated for impairment	16,323	16,739
Outstanding loan balances, Total	16,323	16,739
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	527	758
Allowance for loan losses, Total	$ 527	$ 758	$ 733	$ 245

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The balance of financing receivables that were collectively evaluated for impairment.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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