Loans Held for Investment and Allowance for Loan Losses - Loans Held-for-Investment by Class of Financing Receivables (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	$ 5,286,494		$ 5,171,033	$ 5,293,679
Less: Allowance for credit losses	(63,370)	$ (62,138)	(66,534)	(76,038)	$ (68,947)	$ (52,499)
Net loans held for investment	5,223,124		5,104,499	5,217,641
C&I [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	819,597		1,131,382
Less: Allowance for credit losses	(10,393)	(10,186)	(13,609)
Municipal [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	165,847		181,325
Less: Allowance for credit losses	(315)	(588)	(1,552)
Construction & Development [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	656,530		553,959
Less: Allowance for credit losses	(16,862)	(15,632)	(13,512)
Farm [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	203,064		152,237
Less: Allowance for credit losses	(586)	(607)	(1,876)
Non-Owner Occupied CRE [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	674,958		617,686
Less: Allowance for credit losses	(10,644)	(10,432)	(8,391)
Owner Occupied CRE [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	824,231		746,974
Less: Allowance for credit losses	(10,889)	(10,399)	(12,347)
Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	1,328,798		1,248,409
Less: Allowance for credit losses	(10,057)	(11,234)	(12,601)
Auto [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	394,072		353,595
Less: Allowance for credit losses	(960)	(1,266)	(1,020)
Non-Auto [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	120,450		90,602
Less: Allowance for credit losses	(375)	(482)	(371)
Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	985,444		1,312,707	1,488,345
Less: Allowance for credit losses				(20,191)	(18,572)	(12,122)
Commercial [Member] | C&I [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	819,597		1,131,382
Commercial [Member] | Municipal [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	165,847		181,325
Agriculture [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	98,947		94,864	93,972
Less: Allowance for credit losses	(2,289)	$ (1,312)	(1,255)	(2,331)	(2,544)	(1,206)
Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	3,687,581		3,319,265	3,287,605
Less: Allowance for credit losses				(47,125)	(42,623)	(33,974)
Real Estate [Member] | Construction & Development [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	656,530		553,959
Real Estate [Member] | Farm [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	203,064		152,237
Real Estate [Member] | Non-Owner Occupied CRE [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	674,958		617,686
Real Estate [Member] | Owner Occupied CRE [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	824,231		746,974
Real Estate [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	1,328,798		1,248,409
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	514,522		444,197	423,757
Less: Allowance for credit losses				$ (6,391)	$ (5,208)	$ (5,197)
Consumer [Member] | Auto [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	394,072		353,595
Consumer [Member] | Non-Auto [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total Loans	$ 120,450		$ 90,602