Allowance for Loan Losses - Aging Analysis of Past Due Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 3,726	$ 6,421
Current	1,176,137	1,158,025
Total Loans	1,180,032	1,164,661
Past Due 90 Days and Accruing	34	16
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,074	3,337
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	602	431
90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,050	2,653
Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Purchased Credit-Impaired Loans	169	215
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	849	1,262
Current	152,832	151,124
Total Loans	153,742	152,473
Commercial and Agriculture [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	186	575
Commercial and Agriculture [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		2
Commercial and Agriculture [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	663	685
Commercial and Agriculture [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Purchased Credit-Impaired Loans	61	87
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	823	1,485
Current	168,922	162,614
Total Loans	169,745	164,099
Commercial Real Estate Owner Occupied [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	176	897
Commercial Real Estate Owner Occupied [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	195	104
Commercial Real Estate Owner Occupied [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	452	484
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	155	603
Current	430,940	425,020
Total Loans	431,095	425,623
Commercial Real Estate Non Owner Occupied [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		133
Commercial Real Estate Non Owner Occupied [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	155	470
Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,280	2,627
Current	269,819	265,980
Total Loans	271,207	268,735
Residential Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	440	1,613
Residential Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	307	229
Residential Real Estate [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	533	785
Residential Real Estate [Member] | Receivables Acquired with Deteriorated Credit Quality [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Purchased Credit-Impaired Loans	108	128
Real Estate Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	27	27
Current	100,785	97,504
Total Loans	100,812	97,531
Real Estate Construction [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	27	27
Farm Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	408	213
Current	37,437	39,248
Total Loans	37,845	39,461
Farm Real Estate [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	149	27
Farm Real Estate [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	78
Farm Real Estate [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	181	186
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	184	204
Current	15,402	16,535
Total Loans	15,586	16,739
Past Due 90 Days and Accruing	34	16
Consumer and Other [Member] | 30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	123	92
Consumer and Other [Member] | 60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	22	96
Consumer and Other [Member] | 90 Days or Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 39	$ 16