Loans - Contractual Aging of Recorded Investment in Past Due Loans by Portfolio Classification of Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 1,876	$ 737
Current	1,100,087	845,637
Loans after net deferred loan fees and costs	1,101,963	846,374	$ 834,737
Recorded Investment > 90 Days and Accruing	0	0	0
30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	995	427
60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	146	48
Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	735	262
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,686	620
Current	158,803	79,612
Loans after net deferred loan fees and costs	160,489	80,232	84,052
Consumer Real Estate [Member] | 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	844	342
Consumer Real Estate [Member] | 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	142	24
Consumer Real Estate [Member] | Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	700	254
Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current	192,887	68,588
Loans after net deferred loan fees and costs	192,887	68,588	67,569
Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	74
Current	114,969	108,616
Loans after net deferred loan fees and costs	115,043	108,616	99,954
Agricultural [Member] | 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	74
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current	440,488	419,131
Loans after net deferred loan fees and costs	440,488	419,131	414,666
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current	145,360	127,752
Loans after net deferred loan fees and costs	145,360	127,752	129,865
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	116	117
Current	47,580	41,938
Loans after net deferred loan fees and costs	47,696	42,055	$ 38,631
Consumer [Member] | 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	77	85
Consumer [Member] | 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4	24
Consumer [Member] | Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 35	$ 8