Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	$ 17,186		$ 20,637	$ 3,046
Loans collectively evaluated for credit losses	44,952		45,897	65,901
Total allowance for loan losses	62,138	$ 62,974	66,534	68,947	$ 60,440	$ 52,499
C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	4,588		5,391
Loans collectively evaluated for credit losses	5,598		8,218
Total allowance for loan losses	10,186	12,323	13,609
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	275		1,435
Loans collectively evaluated for credit losses	313		117
Total allowance for loan losses	588	2,191	1,552
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	1,255		982
Loans collectively evaluated for credit losses	57		273
Total allowance for loan losses	1,312		1,255
Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	1,099		652
Loans collectively evaluated for credit losses	14,533		12,860
Total allowance for loan losses	15,632	13,246	13,512
Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	9		1,312
Loans collectively evaluated for credit losses	598		564
Total allowance for loan losses	607	1,223	1,876
Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	4,200		1,921
Loans collectively evaluated for credit losses	6,232		6,470
Total allowance for loan losses	10,432	9,492	8,391
Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,144		5,829
Loans collectively evaluated for credit losses	7,255		6,518
Total allowance for loan losses	10,399	9,878	12,347
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	2,609		3,107
Loans collectively evaluated for credit losses	8,625		9,494
Total allowance for loan losses	11,234	11,189	12,601
Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	5		6
Loans collectively evaluated for credit losses	1,261		1,014
Total allowance for loan losses	1,266	1,068	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	480		369
Total allowance for loan losses	482	379	371
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses				1,250
Loans collectively evaluated for credit losses				17,322
Total allowance for loan losses				18,572	11,773	12,122
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses				83
Loans collectively evaluated for credit losses				2,461
Total allowance for loan losses	$ 1,312	$ 1,985	$ 1,255	2,544	2,154	1,206
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses				1,711
Loans collectively evaluated for credit losses				40,912
Total allowance for loan losses				42,623	41,256	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				5,206
Total allowance for loan losses				$ 5,208	$ 5,257	$ 5,197