Allowance for Loan Losses - Credit Exposures by Internally Assigned Grades (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	$ 953,619	$ 945,123
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	922,400	899,674
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	12,765	25,147
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	18,454	20,302
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	153,742	152,473
Commercial and Agriculture [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	148,387	140,842
Commercial and Agriculture [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,297	8,412
Commercial and Agriculture [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	3,058	3,219
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	169,745	164,099
Commercial Real Estate Owner Occupied [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	163,160	155,756
Commercial Real Estate Owner Occupied [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,256	1,166
Commercial Real Estate Owner Occupied [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	5,329	7,177
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	431,095	425,623
Commercial Real Estate Non Owner Occupied [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	428,084	422,363
Commercial Real Estate Non Owner Occupied [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,288	2,321
Commercial Real Estate Non Owner Occupied [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	723	939
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	67,197	70,498
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	60,275	62,628
Residential Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,831	1,997
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	5,091	5,873
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	92,709	91,587
Real Estate Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	91,406	91,545
Real Estate Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	14	15
Real Estate Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,289	27
Farm Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	37,845	39,461
Farm Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	29,816	25,228
Farm Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	5,079	11,236
Farm Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,950	2,997
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,286	1,382
Consumer and Other [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,272	1,312
Consumer and Other [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	$ 14	$ 70