Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	$ 16,350		$ 20,637	$ 3,241
Loans collectively evaluated for credit losses	47,020		45,897	72,797
Total allowance for loan losses	63,370	$ 62,138	66,534	76,038	$ 68,947	$ 52,499
C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,906		5,391
Loans collectively evaluated for credit losses	6,487		8,218
Total allowance for loan losses	10,393	10,186	13,609
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses			1,435
Loans collectively evaluated for credit losses	315		117
Total allowance for loan losses	315	588	1,552
Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	164		652
Loans collectively evaluated for credit losses	16,698		12,860
Total allowance for loan losses	16,862	15,632	13,512
Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	6		1,312
Loans collectively evaluated for credit losses	580		564
Total allowance for loan losses	586	607	1,876
Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,975		1,921
Loans collectively evaluated for credit losses	6,669		6,470
Total allowance for loan losses	10,644	10,432	8,391
Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,481		5,829
Loans collectively evaluated for credit losses	7,408		6,518
Total allowance for loan losses	10,889	10,399	12,347
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	2,663		3,107
Loans collectively evaluated for credit losses	7,394		9,494
Total allowance for loan losses	10,057	11,234	12,601
Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	4		6
Loans collectively evaluated for credit losses	956		1,014
Total allowance for loan losses	960	1,266	1,020
Non-Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	2		2
Loans collectively evaluated for credit losses	373		369
Total allowance for loan losses	375	482	371
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses				1,234
Loans collectively evaluated for credit losses				18,957
Total allowance for loan losses				20,191	18,572	12,122
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	2,149		982	100
Loans collectively evaluated for credit losses	140		273	2,231
Total allowance for loan losses	$ 2,289	$ 1,312	$ 1,255	2,331	2,544	1,206
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses				1,905
Loans collectively evaluated for credit losses				45,220
Total allowance for loan losses				47,125	42,623	33,974
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses				2
Loans collectively evaluated for credit losses				6,389
Total allowance for loan losses				$ 6,391	$ 5,208	$ 5,197