Allowance for Loan Losses - Summary of Nonaccrual Loans Excluding Purchased Credit-Impaired (PCI) Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	$ 4,944	$ 6,132
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	1,004	887
Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	806	1,476
Commercial Real Estate Non Owner Occupied [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	268	711
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	2,589	2,778
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	27	27
Farm Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	186	186
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Total, Non-Accrual Status	$ 64	$ 67