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
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 89,500	$ 55,108	$ 54,238	$ 51,389
Impact of CECL adoption			(3,195)
Initial allowance on loans purchased with credit deterioration			502
Charge- offs	(2,188)	(2,177)	(3,810)	(3,105)
Recoveries	74	239	735	769
Net charge-offs	(2,114)	(1,938)	(3,075)	(2,336)
Provision for credit losses on loans	18,740	2,758	57,656	6,875
Balance at end of period	106,126	55,928	106,126	55,928
Non-consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period		15,232		13,151
Charge- offs		(244)		(401)
Recoveries		49		201
Net charge-offs		(195)		(200)
Provision for credit losses on loans		347		2,433
Balance at end of period		15,384		15,384
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	6,630		5,625
Impact of CECL adoption			(2,806)
Initial allowance on loans purchased with credit deterioration			432
Charge- offs	(24)		(137)
Recoveries	1		2
Net charge-offs	(23)		(135)
Provision for credit losses on loans	1,287		4,778
Balance at end of period	7,894		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	9,483		8,358
Impact of CECL adoption			(5,507)
Charge- offs	(87)		(87)
Net charge-offs	(87)		(87)
Provision for credit losses on loans	5,508		12,140
Balance at end of period	14,904		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	1,755		2,214
Impact of CECL adoption			(1,056)
Charge- offs			(59)
Recoveries	3		6
Net charge-offs	3		(53)
Provision for credit losses on loans	(169)		484
Balance at end of period	1,589		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	2,259		1,933
Impact of CECL adoption			(778)
Charge- offs	(368)		(397)
Net charge-offs	(368)		(397)
Provision for credit losses on loans	769		1,902
Balance at end of period	2,660		2,660
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	8,553		8,692
Impact of CECL adoption			(3,831)
Initial allowance on loans purchased with credit deterioration			7
Charge- offs	(133)		(351)
Recoveries	5		11
Net charge-offs	(128)		(340)
Provision for credit losses on loans	1,839		5,736
Balance at end of period	10,264		10,264
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,720	11,721	2,767	10,673
Impact of CECL adoption			(1,408)
Charge- offs	(84)	(29)	(116)	(224)
Recoveries	1	2	29	29
Net charge-offs	(83)	(27)	(87)	(195)
Provision for credit losses on loans	219	507	1,584	1,723
Balance at end of period	2,856	12,201	2,856	12,201
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period		6,887		6,328
Charge- offs		(2)		(11)
Recoveries		1		2
Net charge-offs		(1)		(9)
Provision for credit losses on loans		72		639
Balance at end of period		6,958		6,958
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period		11,287		11,027
Charge- offs		(35)		(57)
Recoveries		33		34
Net charge-offs		(2)		(23)
Provision for credit losses on loans		478		759
Balance at end of period		11,763		11,763
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period		3,325		3,261
Charge- offs		(63)		(130)
Recoveries		2		11
Net charge-offs		(61)		(119)
Provision for credit losses on loans		97		219
Balance at end of period		3,361		3,361
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	44,505		15,345
Impact of CECL adoption			11,849
Initial allowance on loans purchased with credit deterioration			62
Charge- offs	(594)		(968)
Recoveries	13		96
Net charge-offs	(581)		(872)
Provision for credit losses on loans	5,518		23,058
Balance at end of period	49,442		49,442
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	4,714	3,234	3,252	3,065
Impact of CECL adoption			(622)
Charge- offs	(195)	(287)	(751)	(569)
Recoveries	43	66	157	175
Net charge-offs	(152)	(221)	(594)	(394)
Provision for credit losses on loans	539	261	3,065	603
Balance at end of period	5,101	3,274	5,101	3,274
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,513	2,449	2,632	2,423
Impact of CECL adoption			(116)
Charge- offs	(100)		(100)
Recoveries	8	9	10	87
Net charge-offs	(92)	9	(90)	87
Provision for credit losses on loans	800	189	795	137
Balance at end of period	3,221	2,647	3,221	2,647
Acquired Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period		973		1,461
Charge- offs		(1,517)		(1,713)
Recoveries		53		206
Net charge-offs		(1,464)		(1,507)
Provision for credit losses on loans		767		322
Balance at end of period		276		276
Farmland [Member] | Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,511		2,821
Impact of CECL adoption			(1,408)
Initial allowance on loans purchased with credit deterioration			1
Charge- offs	(3)		(3)
Net charge-offs	(3)		(3)
Provision for credit losses on loans	1,902		2,999
Balance at end of period	4,410		4,410
Pegasus Bank [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,857		599
Impact of CECL adoption			2,488
Charge- offs	(600)		(841)
Recoveries		24	424	24
Net charge-offs	(600)	24	(417)	24
Provision for credit losses on loans	528	40	1,115	40
Balance at end of period	$ 3,785	$ 64	$ 3,785	$ 64