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, 2021	Mar. 31, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 91,366	$ 54,238
Impact of CECL adoption	(3,200)	(3,195)
Initial allowance on loans purchased with credit deterioration		502
Charge- offs	(666)	(1,160)
Recoveries	160	112
Net charge-offs	(506)	(1,048)
Provision for credit losses on loans		19,583
Balance at end of period	90,860	70,080
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	7,035	5,625
Impact of CECL adoption		(2,806)
Initial allowance on loans purchased with credit deterioration		432
Provision for credit losses on loans	(440)	1,293
Balance at end of period	6,595	4,544
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	11,842	8,358
Impact of CECL adoption		(5,507)
Charge- offs	(38)
Net charge-offs	(38)
Provision for credit losses on loans	5,151	3,084
Balance at end of period	16,955	5,935
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,560	2,214
Impact of CECL adoption		(1,056)
Charge- offs		(3)
Recoveries	3
Net charge-offs	3	(3)
Provision for credit losses on loans	180	(19)
Balance at end of period	2,743	1,136
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	627	1,933
Impact of CECL adoption		(778)
Charge- offs		(1)
Net charge-offs		(1)
Provision for credit losses on loans	356	464
Balance at end of period	983	1,618
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,570	8,692
Impact of CECL adoption		(3,831)
Initial allowance on loans purchased with credit deterioration		7
Charge- offs	(43)	(152)
Recoveries	15	2
Net charge-offs	(28)	(150)
Provision for credit losses on loans	50	1,474
Balance at end of period	2,592	6,192
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,230	2,767
Impact of CECL adoption		(1,408)
Charge- offs	(16)	(25)
Recoveries	3	27
Net charge-offs	(13)	2
Provision for credit losses on loans	(344)	931
Balance at end of period	1,873	2,292
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	32,400	13,462
Impact of CECL adoption		13,195
Initial allowance on loans purchased with credit deterioration		62
Charge- offs	(104)	(87)
Recoveries	26	17
Net charge-offs	(78)	(70)
Provision for credit losses on loans	363	7,344
Balance at end of period	32,685	33,993
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,377	3,252
Impact of CECL adoption		(622)
Charge- offs	(413)	(321)
Recoveries	112	57
Net charge-offs	(301)	(264)
Provision for credit losses on loans	180	1,019
Balance at end of period	3,256	3,385
Oil and Gas [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	17,851	1,883
Impact of CECL adoption		(1,346)
Provision for credit losses on loans	(5,720)	2,746
Balance at end of period	12,131	3,283
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,182	2,632
Impact of CECL adoption		(116)
Charge- offs	(52)
Recoveries		2
Net charge-offs	(52)	2
Provision for credit losses on loans	60	233
Balance at end of period	3,190	2,751
Farmland [Member] | Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,136	2,821
Impact of CECL adoption		(1,408)
Initial allowance on loans purchased with credit deterioration		1
Provision for credit losses on loans	(59)	374
Balance at end of period	3,077	1,788
Pegasus Bank [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	4,556	599
Impact of CECL adoption		2,488
Charge- offs		(571)
Recoveries	1	7
Net charge-offs	1	(564)
Provision for credit losses on loans	223	640
Balance at end of period	$ 4,780	$ 3,163