Loans Held for Investment and Allowance for Loan Losses - Schedule of loans that are individually evaluated for credit losses (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	$ 56,997	$ 56,997
Recorded Investment With No Allowance	22,090	22,090
Recorded Investment With Allowance	20,583	20,583
Total Recorded Investment	42,673	42,673	$ 165,771	$ 197,451
Related Allowance	3,241	3,241	16,350	20,637
Average Recorded Investment	46,961	47,280
Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			15,891	21,439
Recorded Investment With Allowance			9,319	21,180
Related Allowance			1,558	3,419
Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			140,561	154,832
Related Allowance			14,792	17,218
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	8,226	8,226
Recorded Investment With No Allowance	967	967
Recorded Investment With Allowance	5,948	5,948
Total Recorded Investment	6,915	6,915	22,264	40,083
Related Allowance	1,234	1,234	3,906	6,826
Average Recorded Investment	7,293	8,403
Commercial [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			1,311	1,544
Recorded Investment With Allowance			2,119	3,471
Related Allowance			610	799
Commercial [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			18,834	35,068
Related Allowance			3,296	6,027
Commercial [Member] | C&I [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			22,143	30,644
Related Allowance			3,906	5,391
Commercial [Member] | C&I [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			1,311	1,544
Recorded Investment With Allowance			2,119	3,471
Related Allowance			610	799
Commercial [Member] | C&I [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			18,713	25,629
Related Allowance			3,296	4,592
Commercial [Member] | Municipal [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			121	9,439
Related Allowance				1,435
Commercial [Member] | Municipal [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			121	9,439
Related Allowance				1,435
Agriculture [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	1,195	1,195
Recorded Investment With No Allowance	378	378
Recorded Investment With Allowance	589	589
Total Recorded Investment	967	967	7,893	6,648
Related Allowance	100	100	2,149	982
Average Recorded Investment	1,026	1,114
Agriculture [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			328	470
Recorded Investment With Allowance			621	606
Related Allowance			184	96
Agriculture [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			6,944	5,572
Related Allowance			1,965	886
Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	46,973	46,973
Recorded Investment With No Allowance	20,739	20,739
Recorded Investment With Allowance	13,579	13,579
Total Recorded Investment	34,318	34,318	133,411	148,256
Related Allowance	1,905	1,905	10,289	12,821
Average Recorded Investment	38,128	37,216
Real Estate [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			14,252	19,425
Recorded Investment With Allowance			5,894	16,602
Related Allowance			762	2,522
Real Estate [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			113,265	112,229
Related Allowance			9,527	10,299
Real Estate [Member] | Construction & Development [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			6,327	15,205
Related Allowance			164	652
Real Estate [Member] | Construction & Development [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			1,225	1,176
Recorded Investment With Allowance			230	2,661
Related Allowance			6	35
Real Estate [Member] | Construction & Development [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			4,872	11,368
Related Allowance			158	617
Real Estate [Member] | Farm [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			3,105	10,648
Related Allowance			6	1,312
Real Estate [Member] | Farm [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			878	2,614
Recorded Investment With Allowance			87	4,685
Related Allowance				654
Real Estate [Member] | Farm [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			2,140	3,349
Related Allowance			6	658
Real Estate [Member] | Non-Owner Occupied CRE [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			36,158	22,626
Related Allowance			3,975	1,921
Real Estate [Member] | Non-Owner Occupied CRE [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			2,629	4,009
Recorded Investment With Allowance			147	1,234
Related Allowance			16	500
Real Estate [Member] | Non-Owner Occupied CRE [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			33,382	17,383
Related Allowance			3,959	1,421
Real Estate [Member] | Owner Occupied CRE [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			49,332	62,730
Related Allowance			3,481	5,829
Real Estate [Member] | Owner Occupied CRE [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			5,886	7,279
Recorded Investment With Allowance			665	3,518
Related Allowance			99	657
Real Estate [Member] | Owner Occupied CRE [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			42,781	51,933
Related Allowance			3,382	5,172
Real Estate [Member] | Residential [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			38,489	37,047
Related Allowance			2,663	3,107
Real Estate [Member] | Residential [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With No Allowance			3,634	4,347
Recorded Investment With Allowance			4,765	4,504
Related Allowance			641	676
Real Estate [Member] | Residential [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			30,090	28,196
Related Allowance			2,022	2,431
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Contractual Principal Balance	603	603
Recorded Investment With No Allowance	6	6
Recorded Investment With Allowance	467	467
Total Recorded Investment	473	473	2,203	2,464
Related Allowance	2	2	6	8
Average Recorded Investment	$ 514	$ 547
Consumer [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With Allowance			685	501
Related Allowance			2	2
Consumer [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			1,518	1,963
Related Allowance			4	6
Consumer [Member] | Auto [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			1,770	1,930
Related Allowance			4	6
Consumer [Member] | Auto [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With Allowance			589	407
Related Allowance			2	1
Consumer [Member] | Auto [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			1,181	1,523
Related Allowance			2	5
Consumer [Member] | Non-Auto [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			433	534
Related Allowance			2	2
Consumer [Member] | Non-Auto [Member] | Collateral Pledged [Member]
Financing Receivable, Impaired [Line Items]
Recorded Investment With Allowance			96	94
Related Allowance				1
Consumer [Member] | Non-Auto [Member] | Uncollateralized [Member]
Financing Receivable, Impaired [Line Items]
Total Recorded Investment			337	440
Related Allowance			$ 2	$ 1