Loans - Recorded Investment in Nonaccrual Loans by Portfolio Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	$ 1,729	$ 1,384	$ 1,132
Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	101	132
Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	193
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	1,176	1,091
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	98
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	9
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Investment in nonaccrual loans	$ 152	$ 161