Loans and Allowance for Loan Losses - Schedule of How Loans Were Modified as Troubled Debt Restructured Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 441	$ 290
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		146
Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	441	144
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		234
Commercial [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		90
Commercial [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		144
Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	4
Agriculture [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	4
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	363	56
Real Estate [Member] | Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		$ 56
Real Estate [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	363
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	74
Consumer [Member] | Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 74