Allowance for Loan Losses - Credit Exposures by Internally Assigned Grades (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	$ 867,135	$ 854,479
Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	831,221	816,760
Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	17,679	17,342
Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	18,235	20,377
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	139,449	135,462
Commercial and Agriculture [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	131,626	127,867
Commercial and Agriculture [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	4,941	4,300
Commercial and Agriculture [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,882	3,295
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	167,904	161,364
Commercial Real Estate Owner Occupied [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	156,728	151,659
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	5,377	4,016
Commercial Real Estate Owner Occupied [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	5,799	5,689
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	400,411	395,931
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	397,993	393,592
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	1,885	1,676
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	533	663
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	70,608	67,587
Residential Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	62,235	59,015
Residential Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,628	1,661
Residential Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	6,745	6,911
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	48,833	50,721
Real Estate Construction [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	48,790	50,678
Real Estate Construction [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	16	16
Real Estate Construction [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	27	27
Farm Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	38,035	41,170
Farm Real Estate [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	32,035	31,814
Farm Real Estate [Member] | Special Mention [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	3,832	5,673
Farm Real Estate [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	2,168	3,683
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,895	2,244
Consumer and Other [Member] | Pass [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	1,814	2,135
Consumer and Other [Member] | Substandard [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans with credit exposures as assigned an internal risk grade	$ 81	$ 109