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		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 105	$ 779	$ 2,255	$ 1,535
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	0	667	361	1,151
Commercial [Member] | C&I [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			361
Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			68	134
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	77	112	1,766	236
Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			200
Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			1,047
Real Estate [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	77		519
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	28		60	14
Consumer [Member] | Auto [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	19		51
Consumer [Member] | Non Auto [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	9		9
Maturity Extended [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	18	658	743	1,066
Maturity Extended [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	0	658	212	918
Maturity Extended [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			212
Maturity Extended [Member] | Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			68	134
Maturity Extended [Member] | Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	18		463
Maturity Extended [Member] | Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			200
Maturity Extended [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	18		263
Maturity Extended [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	87	121	1,512	469
Combined Rate and Maturity [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans		9	149	233
Combined Rate and Maturity [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			149
Combined Rate and Maturity [Member] | Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	59	$ 112	1,303	$ 236
Combined Rate and Maturity [Member] | Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans			1,047
Combined Rate and Maturity [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	59		256
Combined Rate and Maturity [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	28		60
Combined Rate and Maturity [Member] | Consumer [Member] | Auto [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	19		51
Combined Rate and Maturity [Member] | Consumer [Member] | Non Auto [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans	$ 9		$ 9