Loans Held for Investment and Allowance for Loan Losses - Schedule of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 5,286,000	$ 5,171,000
Total Loans	5,286,494	5,171,033	$ 5,293,679
Total 90 Days Past Due Still Accruing	23	113	23
C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	820,000	1,131,000
Total Loans	819,597	1,131,382
Municipal [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	166,000	181,000
Total Loans	165,847	181,325
Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	656,000	554,000
Total Loans	656,530	553,959
Farm [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	203,000	152,000
Total Loans	203,064	152,237
Non-Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	675,000	618,000
Total Loans	674,958	617,686
Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	824,000	747,000
Total Loans	824,231	746,974
Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,329,000	1,248,000
Total Loans	1,328,798	1,248,409
Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	394,000	354,000
Total Loans	394,072	353,595
Non-Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	120,000	91,000
Total Loans	120,450	90,602
Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	985,444	1,312,707	1,488,345
Total 90 Days Past Due Still Accruing	23	21
Commercial [Member] | C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	819,597	1,131,382
Total 90 Days Past Due Still Accruing	23	21
Commercial [Member] | Municipal [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	165,847	181,325
Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	98,947	94,864	93,972
Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	3,687,581	3,319,265	3,287,605
Total 90 Days Past Due Still Accruing		33	1
Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	656,530	553,959
Real Estate [Member] | Farm [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	203,064	152,237
Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	674,958	617,686
Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	824,231	746,974
Real Estate [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	1,328,798	1,248,409
Total 90 Days Past Due Still Accruing		33
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	514,522	444,197	423,757
Total 90 Days Past Due Still Accruing		59	22
Consumer [Member] | Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	394,072	353,595
Total 90 Days Past Due Still Accruing		59
Consumer [Member] | Non-Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	120,450	90,602
15-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	19,358	20,166	16,714
15-59 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,136	3,647	5,165
15-59 Days Past Due [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,136	3,647
15-59 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	115	193	103
15-59 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	14,725	15,561	10,905
15-59 Days Past Due [Member] | Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,676	4,775
15-59 Days Past Due [Member] | Real Estate [Member] | Farm [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,182	708
15-59 Days Past Due [Member] | Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	800	613
15-59 Days Past Due [Member] | Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	260	1,393
15-59 Days Past Due [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7,807	8,072
15-59 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	382	765	541
15-59 Days Past Due [Member] | Consumer [Member] | Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	323	551
15-59 Days Past Due [Member] | Consumer [Member] | Non-Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	59	214
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,439	1,067	606
60-89 Days Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	516	406	476
60-89 Days Past Due [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	516	406
60-89 Days Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,074	95	50
60-89 Days Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	803	384	0
60-89 Days Past Due [Member] | Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due		44
60-89 Days Past Due [Member] | Real Estate [Member] | Farm [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	13
60-89 Days Past Due [Member] | Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0
60-89 Days Past Due [Member] | Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	315	322
60-89 Days Past Due [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	475	18
60-89 Days Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	46	182	80
60-89 Days Past Due [Member] | Consumer [Member] | Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	43	158
60-89 Days Past Due [Member] | Consumer [Member] | Non-Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3	24
Greater than 90 Days [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,839	1,059	459
Greater than 90 Days [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	666	576	116
Greater than 90 Days [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	666	576
Greater than 90 Days [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	100		6
Greater than 90 Days [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,066	408	306
Greater than 90 Days [Member] | Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	114
Greater than 90 Days [Member] | Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,537	133
Greater than 90 Days [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	415	275
Greater than 90 Days [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7	75	31
Greater than 90 Days [Member] | Consumer [Member] | Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	7	75
Total Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	24,636	22,292	17,779
Total Past Due [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,318	4,629	5,757
Total Past Due [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,318	4,629
Total Past Due [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,289	288	159
Total Past Due [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	17,594	16,353	11,211
Total Past Due [Member] | Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,790	4,819
Total Past Due [Member] | Real Estate [Member] | Farm [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,195	708
Total Past Due [Member] | Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	800	613
Total Past Due [Member] | Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,112	1,848
Total Past Due [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,697	8,365
Total Past Due [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	435	1,022	652
Total Past Due [Member] | Consumer [Member] | Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	373	784
Total Past Due [Member] | Consumer [Member] | Non-Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	62	238
Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	5,261,858	5,148,741	5,275,900
Current [Member] | Commercial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	980,126	1,308,078	1,482,588
Current [Member] | Commercial [Member] | C&I [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	814,279	1,126,753
Current [Member] | Commercial [Member] | Municipal [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	165,847	181,325
Current [Member] | Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	97,658	94,576	93,813
Current [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,669,987	3,302,912	3,276,394
Current [Member] | Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	651,740	549,140
Current [Member] | Real Estate [Member] | Farm [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	201,869	151,529
Current [Member] | Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	674,158	617,073
Current [Member] | Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	822,119	745,126
Current [Member] | Real Estate [Member] | Residential [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,320,101	1,240,044
Current [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	514,087	443,175	$ 423,105
Current [Member] | Consumer [Member] | Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	393,699	352,811
Current [Member] | Consumer [Member] | Non-Auto [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 120,388	$ 90,364