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		9 Months Ended
	Jan. 01, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 54,238	$ 83,963	$ 89,500	$ 91,366	$ 54,238
Impact of CECL adoption	(3,200)				(3,195)
Initial allowance on loans purchased with credit deterioration		1,027		8,299	502
Charge- offs		(300)	(2,188)	(5,414)	(3,810)
Recoveries		290	74	678	735
Net charge-offs		(10)	(2,114)	(4,736)	(3,075)
(Benefit from) provision for credit losses		1,483	18,740	(8,466)	57,656
Balance at end of period		86,463	106,126	86,463	106,126
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	5,625	6,755	6,630	7,035	5,625
Impact of CECL adoption					(2,806)
Initial allowance on loans purchased with credit deterioration		93		1,080	432
Charge- offs		(3)	(24)	(3)	(137)
Recoveries		72	1	73	2
Net charge-offs		69	(23)	70	(135)
(Benefit from) provision for credit losses		(51)	1,287	(1,319)	4,778
Balance at end of period		6,866	7,894	6,866	7,894
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	8,358	14,490	9,483	11,842	8,358
Impact of CECL adoption					(5,507)
Initial allowance on loans purchased with credit deterioration		191		824
Charge- offs		(7)	(87)	(803)	(87)
Recoveries		67		67
Net charge-offs		60	(87)	(736)	(87)
(Benefit from) provision for credit losses		(6)	5,508	2,805	12,140
Balance at end of period		14,735	14,904	14,735	14,904
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,214	2,893	1,755	2,560	2,214
Impact of CECL adoption					(1,056)
Initial allowance on loans purchased with credit deterioration		0		173
Charge- offs					(59)
Recoveries		4	3	9	6
Net charge-offs		4	3	9	(53)
(Benefit from) provision for credit losses		398	(169)	553	484
Balance at end of period		3,295	1,589	3,295	1,589
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	1,933	889	2,259	627	1,933
Impact of CECL adoption					(778)
Charge- offs			(368)		(397)
Net charge-offs			(368)		(397)
(Benefit from) provision for credit losses		(35)	769	227	1,902
Balance at end of period		854	2,660	854	2,660
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	8,692	2,805	8,553	2,570	8,692
Impact of CECL adoption					(3,831)
Initial allowance on loans purchased with credit deterioration		9		126	7
Charge- offs		(4)	(133)	(56)	(351)
Recoveries		9	5	36	11
Net charge-offs		5	(128)	(20)	(340)
(Benefit from) provision for credit losses		186	1,839	329	5,736
Balance at end of period		3,005	10,264	3,005	10,264
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,767	1,941	2,720	2,230	2,767
Impact of CECL adoption					(1,408)
Charge- offs		0	(84)	(46)	(116)
Recoveries		46	1	50	29
Net charge-offs		46	(83)	4	(87)
(Benefit from) provision for credit losses		202	219	(45)	1,584
Balance at end of period		2,189	2,856	2,189	2,856
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	13,462	31,615	34,036	32,400	13,462
Impact of CECL adoption					13,195
Initial allowance on loans purchased with credit deterioration		952		5,663	62
Charge- offs		(146)	(594)	(3,683)	(968)
Recoveries		43	13	194	96
Net charge-offs		(103)	(581)	(3,489)	(872)
(Benefit from) provision for credit losses		(859)	2,463	(2,969)	10,071
Balance at end of period		31,605	35,918	31,605	35,918
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,252	3,315	4,714	3,377	3,252
Impact of CECL adoption					(622)
Initial allowance on loans purchased with credit deterioration		30		38
Charge- offs		(67)	(195)	(689)	(751)
Recoveries		49	43	247	157
Net charge-offs		(18)	(152)	(442)	(594)
(Benefit from) provision for credit losses		51	539	405	3,065
Balance at end of period		3,378	5,101	3,378	5,101
Oil and Gas [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	1,883	7,817	10,469	17,851	1,883
Impact of CECL adoption					(1,346)
(Benefit from) provision for credit losses		(38)	3,055	(10,072)	12,987
Balance at end of period		7,779	13,524	7,779	13,524
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,632	3,138	2,513	3,182	2,632
Impact of CECL adoption					(116)
Charge- offs		(73)	(100)	(134)	(100)
Recoveries			8		10
Net charge-offs		(73)	(92)	(134)	(90)
(Benefit from) provision for credit losses		239	800	256	795
Balance at end of period		3,304	3,221	3,304	3,221
Farmland [Member] | Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,821	3,715	2,511	3,136	2,821
Impact of CECL adoption					(1,408)
Initial allowance on loans purchased with credit deterioration		(248)		395	1
Charge- offs			(3)		(3)
Recoveries		0		1
Net charge-offs		0	(3)	1	(3)
(Benefit from) provision for credit losses		967	1,902	902	2,999
Balance at end of period		4,434	4,410	4,434	4,410
Pegasus Bank [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 599	4,590	3,857	4,556	599
Impact of CECL adoption					2,488
Charge- offs			(600)		(841)
Recoveries				1	424
Net charge-offs			(600)	1	(417)
(Benefit from) provision for credit losses		429	528	462	1,115
Balance at end of period		$ 5,019	$ 3,785	$ 5,019	$ 3,785