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
	Mar. 31, 2022	Mar. 31, 2021
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 83,936	$ 91,366
Initial allowance on loans purchased with credit deterioration	78
Charge- offs	(322)	(666)
Recoveries	611	160
Net charge-offs	289	(506)
Provision for/(benefit from) credit losses	2,936	0
Balance at end of period	87,239	90,860
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	6,410	6,911
Charge- offs	(16)
Recoveries	48
Net charge-offs	32
Provision for/(benefit from) credit losses	346	(406)
Balance at end of period	6,788	6,505
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	16,987	12,318
Charge- offs		(38)
Net charge-offs		(38)
Provision for/(benefit from) credit losses	3,687	5,219
Balance at end of period	20,674	17,499
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,490	2,723
Recoveries	3	3
Net charge-offs	3	3
Provision for/(benefit from) credit losses	(184)	217
Balance at end of period	3,309	2,943
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	1,092	726
Provision for/(benefit from) credit losses	1,072	366
Balance at end of period	2,164	1,092
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,076	2,822
Initial allowance on loans purchased with credit deterioration	2
Charge- offs	(44)	(43)
Recoveries	7	15
Net charge-offs	(37)	(28)
Provision for/(benefit from) credit losses	380	143
Balance at end of period	3,421	2,937
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,104	2,236
Charge- offs		(16)
Recoveries	402	3
Net charge-offs	402	(13)
Provision for/(benefit from) credit losses	(399)	(343)
Balance at end of period	2,107	1,880
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	26,073	33,020
Initial allowance on loans purchased with credit deterioration	48
Charge- offs	(171)	(104)
Recoveries	113	27
Net charge-offs	(58)	(77)
Provision for/(benefit from) credit losses	(77)	922
Balance at end of period	25,986	33,865
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,734	3,542
Initial allowance on loans purchased with credit deterioration	28
Charge- offs	(80)	(413)
Recoveries	38	112
Net charge-offs	(42)	(301)
Provision for/(benefit from) credit losses	51	180
Balance at end of period	3,771	3,421
Oil and Gas [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	12,978	20,733
Provision for/(benefit from) credit losses	(1,573)	(6,293)
Balance at end of period	11,405	14,440
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,170	3,182
Charge- offs	(11)	(52)
Net charge-offs	(11)	(52)
Provision for/(benefit from) credit losses	72	60
Balance at end of period	3,231	3,190
Farmland [Member] | Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	4,822	3,153
Provision for/(benefit from) credit losses	(439)	(65)
Balance at end of period	$ 4,383	$ 3,088