Loans Held for Investment and Allowance for Credit Losses on Loans - Activity in Allowance for Credit Losses on Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 54,238	$ 51,389
Impact of CECL adoption	(3,195)
Initial allowance on loans purchased with credit deterioration	502
Charge- offs	(23,832)	(6,386)
Recoveries	1,005	948
Net charge-offs	(22,827)	(5,438)
Provision for credit losses on loans	62,648	8,287
Balance at end of period	91,366	54,238
Non-consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	15,341	13,151
Charge- offs		(514)
Recoveries		272
Net charge-offs		(242)
Provision for credit losses on loans		2,432
Balance at end of period		15,341
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	5,625
Impact of CECL adoption	(2,806)
Initial allowance on loans purchased with credit deterioration	432
Charge- offs	(773)
Recoveries	10
Net charge-offs	(763)
Provision for credit losses on loans	4,547
Balance at end of period	7,035	5,625
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
Impact of CECL adoption	(5,507)
Charge- offs	(3,609)
Net charge-offs	(3,609)
Provision for credit losses on loans	12,600
Balance at end of period	11,842	8,358
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
Impact of CECL adoption	(1,056)
Charge- offs	(59)
Recoveries	123
Net charge-offs	64
Provision for credit losses on loans	1,338
Balance at end of period	2,560	2,214
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
Impact of CECL adoption	(778)
Charge- offs	(29)
Net charge-offs	(29)
Provision for credit losses on loans	(499)
Balance at end of period	627	1,933
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	8,692
Impact of CECL adoption	(3,831)
Initial allowance on loans purchased with credit deterioration	7
Charge- offs	(465)
Recoveries	44
Net charge-offs	(421)
Provision for credit losses on loans	(1,877)
Balance at end of period	2,570	8,692
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,767
Impact of CECL adoption	(1,408)
Charge- offs	(126)
Recoveries	54
Net charge-offs	(72)
Provision for credit losses on loans	943
Balance at end of period	2,230	2,767
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	7,040	6,328
Charge- offs		(34)
Recoveries		25
Net charge-offs		(9)
Provision for credit losses on loans		721
Balance at end of period		7,040
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	11,157	11,027
Charge- offs		(67)
Recoveries		42
Net charge-offs		(25)
Provision for credit losses on loans		155
Balance at end of period		11,157
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,387	3,261
Charge- offs		(146)
Recoveries		22
Net charge-offs		(124)
Provision for credit losses on loans		250
Balance at end of period		3,387
Real Estate [Member] | Residential Real Estate Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	10,149	10,673
Charge- offs		(2,697)
Recoveries		30
Net charge-offs		(2,667)
Provision for credit losses on loans		2,143
Balance at end of period		10,149
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	13,462
Impact of CECL adoption	13,195
Initial allowance on loans purchased with credit deterioration	62
Charge- offs	(3,320)
Recoveries	116
Net charge-offs	(3,204)
Provision for credit losses on loans	8,885
Balance at end of period	32,400	13,462
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,252
Impact of CECL adoption	(622)
Charge- offs	(1,142)
Recoveries	224
Net charge-offs	(918)
Provision for credit losses on loans	1,665
Balance at end of period	3,377	3,252
Oil and Gas [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	1,883
Impact of CECL adoption	(1,346)
Charge- offs	(11,245)
Net charge-offs	(11,245)
Provision for credit losses on loans	28,559
Balance at end of period	17,851	1,883
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,632	2,423
Impact of CECL adoption	(116)
Charge- offs	(168)
Recoveries	10	90
Net charge-offs	(158)	90
Provision for credit losses on loans	824	119
Balance at end of period	3,182	2,632
Consumer Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,319	3,065
Charge- offs		(868)
Recoveries		218
Net charge-offs		(650)
Provision for credit losses on loans		904
Balance at end of period		3,319
Acquired Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	614	1,461
Charge- offs		(2,060)
Recoveries		219
Net charge-offs		(1,841)
Provision for credit losses on loans		994
Balance at end of period		614
Farmland [Member] | Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,821
Impact of CECL adoption	(1,408)
Initial allowance on loans purchased with credit deterioration	1
Charge- offs	(2,055)
Net charge-offs	(2,055)
Provision for credit losses on loans	3,777
Balance at end of period	3,136	2,821
Pegasus Bank [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	599
Impact of CECL adoption	2,488
Charge- offs	(841)
Recoveries	424	30
Net charge-offs	(417)	30
Provision for credit losses on loans	1,886	569
Balance at end of period	$ 4,556	$ 599