Loans Held for Investment and Allowance for Loan Losses - Schedule of Investment in Loans Related to Balance in Allowance for Loan Losses on Basis of Company's Impairment Methodology (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 165,771	$ 197,451	$ 42,673
Loan collectively evaluated for impairment	5,120,723	4,973,582	5,251,006
Total	5,286,494	5,171,033	5,293,679
C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	22,143	30,644
Loan collectively evaluated for impairment	797,454	1,100,738
Total	819,597	1,131,382
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	121	9,439
Loan collectively evaluated for impairment	165,726	171,886
Total	165,847	181,325
Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	6,327	15,205
Loan collectively evaluated for impairment	650,203	538,754
Total	656,530	553,959
Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	3,105	10,648
Loan collectively evaluated for impairment	199,959	141,589
Total	203,064	152,237
Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	36,158	22,626
Loan collectively evaluated for impairment	638,800	595,060
Total	674,958	617,686
Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	49,332	62,730
Loan collectively evaluated for impairment	774,899	684,244
Total	824,231	746,974
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	38,489	37,047
Loan collectively evaluated for impairment	1,290,309	1,211,362
Total	1,328,798	1,248,409
Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,770	1,930
Loan collectively evaluated for impairment	392,302	351,665
Total	394,072	353,595
Non-Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	433	534
Loan collectively evaluated for impairment	120,017	90,068
Total	120,450	90,602
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			6,915
Loan collectively evaluated for impairment			1,481,430
Total	985,444	1,312,707	1,488,345
Commercial [Member] | C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	819,597	1,131,382
Commercial [Member] | Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	165,847	181,325
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	7,893	6,648	967
Loan collectively evaluated for impairment	91,054	88,216	93,005
Total	98,947	94,864	93,972
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			34,318
Loan collectively evaluated for impairment			3,253,287
Total	3,687,581	3,319,265	3,287,605
Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	656,530	553,959
Real Estate [Member] | Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	203,064	152,237
Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	674,958	617,686
Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	824,231	746,974
Real Estate [Member] | Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	1,328,798	1,248,409
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			473
Loan collectively evaluated for impairment			423,284
Total	514,522	444,197	$ 423,757
Consumer [Member] | Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	394,072	353,595
Consumer [Member] | Non-Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 120,450	$ 90,602