Loans Held for Investment and Allowance for Credit Losses on Loans - Activity in Allowance for Credit Losses on Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 87,239	$ 90,860	$ 83,936	$ 91,366
Initial allowance on loans purchased with credit deterioration		7,272	78	7,272
Charge- offs	(915)	(4,448)	(1,237)	(5,114)
Recoveries	110	228	721	388
Net charge-offs	(805)	(4,220)	(516)	(4,726)
Provision for (benefit from) credit losses	501	(9,949)	3,437	(9,949)
Balance at end of period	86,935	83,963	86,935	83,963
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	8,281	6,505	7,568	8,470
Initial allowance on loans purchased with credit deterioration		987		987
Charge- offs	(4)		(20)
Recoveries	30	1	78	1
Net charge-offs	26	1	58	1
Provision for (benefit from) credit losses	(1,362)	805	(681)	(1,160)
Balance at end of period	6,945	8,298	6,945	8,298
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	20,674	19,115	16,987	12,318
Initial allowance on loans purchased with credit deterioration		633		633
Charge- offs		(758)		(796)
Net charge-offs		(758)		(796)
Provision for (benefit from) credit losses	2,263	(3,952)	5,950	2,883
Balance at end of period	22,937	15,038	22,937	15,038
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,309	2,943	3,490	2,723
Initial allowance on loans purchased with credit deterioration		173		173
Recoveries	2	2	5	5
Net charge-offs	2	2	5	5
Provision for (benefit from) credit losses	417	(23)	233	194
Balance at end of period	3,728	3,095	3,728	3,095
Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,164	1,092	1,092	726
Provision for (benefit from) credit losses	128	(94)	1,200	272
Balance at end of period	2,292	998	2,292	998
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,421	2,937	3,076	2,822
Initial allowance on loans purchased with credit deterioration		117	2	117
Charge- offs	(5)	(9)	(49)	(52)
Recoveries	6	12	13	27
Net charge-offs	1	3	(36)	(25)
Provision for (benefit from) credit losses	(39)	(136)	341	7
Balance at end of period	3,383	2,921	3,383	2,921
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,107	1,880	2,104	2,236
Charge- offs	(36)	(30)	(36)	(46)
Recoveries		1	402	4
Net charge-offs	(36)	(29)	366	(42)
Provision for (benefit from) credit losses	(20)	112	(419)	(231)
Balance at end of period	2,051	1,963	2,051	1,963
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	27,724	35,439	28,085	34,643
Initial allowance on loans purchased with credit deterioration		4,711	48	4,711
Charge- offs	(717)	(3,442)	(899)	(3,598)
Recoveries	30	125	143	152
Net charge-offs	(687)	(3,317)	(756)	(3,446)
Provision for (benefit from) credit losses	796	(2,523)	456	(1,598)
Balance at end of period	27,833	34,310	27,833	34,310
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,771	3,421	3,734	3,542
Initial allowance on loans purchased with credit deterioration		8	28	8
Charge- offs	(153)	(209)	(233)	(622)
Recoveries	42	86	80	198
Net charge-offs	(111)	(123)	(153)	(424)
Provision for (benefit from) credit losses	434	175	485	355
Balance at end of period	4,094	3,481	4,094	3,481
Oil and Gas [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	11,405	14,440	12,978	20,733
Provision for (benefit from) credit losses	(2,098)	(4,308)	(3,671)	(10,601)
Balance at end of period	9,307	10,132	9,307	10,132
Farmland [Member] | Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	4,383	3,088	4,822	3,153
Initial allowance on loans purchased with credit deterioration		643		643
Recoveries		1		1
Net charge-offs		1		1
Provision for (benefit from) credit losses	(18)	(5)	(457)	(70)
Balance at end of period	$ 4,365	$ 3,727	$ 4,365	$ 3,727