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	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	$ 190,416	$ 197,451	$ 39,226
Loan collectively evaluated for impairment	5,132,146	4,973,582	4,600,163
Total	5,322,562	5,171,033	4,639,389
C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	19,289	30,644
Loan collectively evaluated for impairment	1,158,837	1,100,738
Total	1,178,126	1,131,382
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	9,777	9,439
Loan collectively evaluated for impairment	167,172	171,886
Total	176,949	181,325
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	6,859	6,648
Loan collectively evaluated for impairment	83,507	88,216
Total	90,366	94,864
Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	12,497	15,205
Loan collectively evaluated for impairment	575,431	538,754
Total	587,928	553,959
Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	10,295	10,648
Loan collectively evaluated for impairment	151,751	141,589
Total	162,046	152,237
Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	39,387	22,626
Loan collectively evaluated for impairment	610,757	595,060
Total	650,144	617,686
Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	55,450	62,730
Loan collectively evaluated for impairment	704,456	684,244
Total	759,906	746,974
Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	34,695	37,047
Loan collectively evaluated for impairment	1,220,032	1,211,362
Total	1,254,727	1,248,409
Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	1,716	1,930
Loan collectively evaluated for impairment	368,311	351,665
Total	370,027	353,595
Non-Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment	451	534
Loan collectively evaluated for impairment	91,892	90,068
Total	92,343	90,602
Commercial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			5,620
Loan collectively evaluated for impairment			863,830
Total	1,355,075	1,312,707	869,450
Commercial [Member] | C&I [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	1,178,126	1,131,382
Commercial [Member] | Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	176,949	181,325
Agriculture [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			1,128
Loan collectively evaluated for impairment			98,454
Total	90,366	94,864	99,582
Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			32,156
Loan collectively evaluated for impairment			3,217,093
Total	3,414,751	3,319,265	3,249,249
Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	587,928	553,959
Real Estate [Member] | Farm [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	162,046	152,237
Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	650,144	617,686
Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	759,906	746,974
Real Estate [Member] | Residential [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	1,254,727	1,248,409
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loans individually evaluated for impairment			322
Loan collectively evaluated for impairment			420,786
Total	462,370	444,197	$ 421,108
Consumer [Member] | Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	370,027	353,595
Consumer [Member] | Non-Auto [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 92,343	$ 90,602