Loans - Contractual Maturity Schedule by Major Category of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	$ 1,103,096	$ 847,289
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	161,180	80,766
Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	192,903	68,609
Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	114,920	108,495
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	441,200	419,784
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	137,936	121,793
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	47,573	41,953
Major Category Of Receivables Due Within One Year [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	6,370
Major Category Of Receivables Due Within One Year [Member] | Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	680
Major Category Of Receivables Due Within One Year [Member] | Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	71,544
Major Category Of Receivables Due Within One Year [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	18,402
Major Category Of Receivables Due Within One Year [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	74,195
Major Category Of Receivables Due Within One Year [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	6,025
Major Category Of Receivables Due Within One Year to Five Years [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	17,260
Major Category Of Receivables Due Within One Year to Five Years [Member] | Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	6,082
Major Category Of Receivables Due Within One Year to Five Years [Member] | Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	31,632
Major Category Of Receivables Due Within One Year to Five Years [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	169,066
Major Category Of Receivables Due Within One Year to Five Years [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	53,787
Major Category Of Receivables Due Within One Year to Five Years [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	30,870
Major Category Of Receivables Over Five Years [Member] | Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	137,550
Major Category Of Receivables Over Five Years [Member] | Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	186,141
Major Category Of Receivables Over Five Years [Member] | Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	11,744
Major Category Of Receivables Over Five Years [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	253,732
Major Category Of Receivables Over Five Years [Member] | Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	9,954
Major Category Of Receivables Over Five Years [Member] | Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	10,678
Industrial Development Bonds [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	7,384	$ 5,889
Industrial Development Bonds [Member] | Major Category Of Receivables Due Within One Year [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	424
Industrial Development Bonds [Member] | Major Category Of Receivables Due Within One Year to Five Years [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	291
Industrial Development Bonds [Member] | Major Category Of Receivables Over Five Years [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans - Gross	$ 6,669