Allowance for Loan Losses - Ending Allocation of Allowance for Loan Losses and Loan Balances Outstanding (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	$ 194	$ 141
Allowance for loan losses, Collectively evaluated for impairment	13,628	13,530
Allowance for loan losses, Total	13,822	13,679	$ 12,814	$ 13,134
Outstanding loan balances, Individually evaluated for impairment	3,061	2,857
Outstanding loan balances, Collectively evaluated for impairment	1,569,274	1,558,160
Outstanding loan balances, Total	1,573,193	1,561,941
Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration		8
Loan balance of loans acquired with credit deterioration	858	924
Commercial and Agriculture [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	53
Allowance for loan losses, Collectively evaluated for impairment	1,781	1,747
Allowance for loan losses, Total	1,834	1,747	1,359	1,562
Outstanding loan balances, Individually evaluated for impairment	367	367
Outstanding loan balances, Collectively evaluated for impairment	178,798	176,693
Outstanding loan balances, Total	179,200	177,101
Commercial and Agriculture [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan balance of loans acquired with credit deterioration	35	41
Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	10	12
Allowance for loan losses, Collectively evaluated for impairment	1,969	1,950
Allowance for loan losses, Total	1,979	1,962	2,030	2,043
Outstanding loan balances, Individually evaluated for impairment	472	484
Outstanding loan balances, Collectively evaluated for impairment	216,558	209,637
Outstanding loan balances, Total	217,030	210,121
Commercial Real Estate Non Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	5,989	5,803
Allowance for loan losses, Total	5,989	5,803	5,670	5,307
Outstanding loan balances, Individually evaluated for impairment	382	31
Outstanding loan balances, Collectively evaluated for impairment	539,038	523,567
Outstanding loan balances, Total	539,420	523,598
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	124	122
Allowance for loan losses, Collectively evaluated for impairment	1,310	1,401
Allowance for loan losses, Total	1,434	1,531	1,750	1,910
Outstanding loan balances, Individually evaluated for impairment	1,148	1,279
Outstanding loan balances, Collectively evaluated for impairment	459,369	455,688
Outstanding loan balances, Total	461,340	457,850
Residential Real Estate [Member] \| Receivables Acquired with Deteriorated Credit Quality [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans acquired with credit deterioration		8
Loan balance of loans acquired with credit deterioration	823	883
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	971	1,046
Allowance for loan losses, Total	971	1,046	820	834
Outstanding loan balances, Collectively evaluated for impairment	123,905	135,195
Outstanding loan balances, Total	123,905	135,195
Farm Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Individually evaluated for impairment	7	7
Allowance for loan losses, Collectively evaluated for impairment	359	390
Allowance for loan losses, Total	366	397	411	430
Outstanding loan balances, Individually evaluated for impairment	692	696
Outstanding loan balances, Collectively evaluated for impairment	34,953	37,817
Outstanding loan balances, Total	35,645	38,513
Consumer and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	251	284
Allowance for loan losses, Total	251	284	247	290
Outstanding loan balances, Collectively evaluated for impairment	16,653	19,563
Outstanding loan balances, Total	16,653	19,563
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses, Collectively evaluated for impairment	998	909
Allowance for loan losses, Total	$ 998	$ 909	$ 527	$ 758

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For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

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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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