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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 319	$ 375	$ 755	$ 1,548
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	196	122	484	279
Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		253	134	619
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	123		123	650
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			14
Extended Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		375	408	835
Extended Maturity [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		122	260	279
Extended Maturity [Member] | Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		$ 253	134	354
Extended Maturity [Member] | Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans				202
Extended Maturity [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			14
Combined Rate and Maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	319		347	713
Combined Rate and Maturity [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	196		224
Combined Rate and Maturity [Member] | Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans				265
Combined Rate and Maturity [Member] | Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 123		$ 123	$ 448