Loans Held for Investment and Allowance for Loan Losses - Schedule of How Loans Were Modified as Troubled Debt Restructured Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 375	$ 480	$ 1,548	$ 921
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	122	279	279	279
Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	253		619	4
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		162	650	525
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		39		113
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	375	162	835	162
Extended Maturity [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	122		279
Extended Maturity [Member] | Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 253		354
Extended Maturity [Member] | Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		162	202	162
Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		318	713	759
Combined Rate and Maturity [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		279		279
Combined Rate and Maturity [Member] | Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			265	4
Combined Rate and Maturity [Member] | Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			$ 448	363
Combined Rate and Maturity [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		$ 39		$ 113