Loans Held for Investment and Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	$ 18,573	$ 20,637	$ 3,386
Loans collectively evaluated for credit losses	44,401	45,897	57,054
Total allowance for loan losses	62,974	66,534	60,440	$ 52,499
C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,881	5,391
Loans collectively evaluated for credit losses	8,442	8,218
Total allowance for loan losses	12,323	13,609
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	1,536	1,435
Loans collectively evaluated for credit losses	655	117
Total allowance for loan losses	2,191	1,552
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	1,790	982
Loans collectively evaluated for credit losses	195	273
Total allowance for loan losses	1,985	1,255
Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	880	652
Loans collectively evaluated for credit losses	12,366	12,860
Total allowance for loan losses	13,246	13,512
Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	671	1,312
Loans collectively evaluated for credit losses	552	564
Total allowance for loan losses	1,223	1,876
Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,335	1,921
Loans collectively evaluated for credit losses	6,157	6,470
Total allowance for loan losses	9,492	8,391
Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	3,599	5,829
Loans collectively evaluated for credit losses	6,279	6,518
Total allowance for loan losses	9,878	12,347
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses	2,874	3,107
Loans collectively evaluated for credit losses	8,315	9,494
Total allowance for loan losses	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	1,063	1,014
Total allowance for loan losses	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	377	369
Total allowance for loan losses	379	371
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses			1,271
Loans collectively evaluated for credit losses			10,502
Total allowance for loan losses			11,773	12,122
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses			98
Loans collectively evaluated for credit losses			2,056
Total allowance for loan losses	$ 1,985	$ 1,255	2,154	1,206
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses			2,016
Loans collectively evaluated for credit losses			39,240
Total allowance for loan losses			41,256	33,974
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for credit losses			1
Loans collectively evaluated for credit losses			5,256
Total allowance for loan losses			$ 5,257	$ 5,197