Loans Held for Investment and Allowance for Credit Losses on Loans - Age Analysis of Loans Held for Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 45,532	$ 49,793
Current Loans	6,348,974	5,612,350
Total Loans	6,394,506	5,662,143
Accruing Loans 90 Days or More Past Due	4,802	11,834
Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,368
Current Loans	639,619
Total Loans	641,987	621,188
Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	357
Current Loans	970,801
Total Loans	971,158	851,200
Accruing Loans 90 Days or More Past Due	35
Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	597
Current Loans	229,018
Total Loans	229,615	287,138
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,181
Current Loans	165,900
Total Loans	168,081	187,647
Accruing Loans 90 Days or More Past Due	384
Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,388
Current Loans	204,807
Total Loans	206,195	189,480
Accruing Loans 90 Days or More Past Due	282
Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	9,869
Current Loans	843,447
Total Loans	853,316	834,849
Accruing Loans 90 Days or More Past Due	945
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		7,726
Current Loans		699,690
Total Loans		707,416
Accruing Loans 90 Days or More Past Due		3,799
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		2,199
Current Loans		1,134,976
Total Loans		1,137,175
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		7,893
Current Loans		332,679
Total Loans		340,572
Accruing Loans 90 Days or More Past Due		1,660
Real Estate [Member] | Residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		8,065
Current Loans		912,767
Total Loans		920,832
Accruing Loans 90 Days or More Past Due		549
Real Estate [Member] | Farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	7,864
Current Loans	245,094
Total Loans	252,958	246,988
Accruing Loans 90 Days or More Past Due	135
Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	3,189
Current Loans	352,216
Total Loans	355,405	359,529
Accruing Loans 90 Days or More Past Due	386
Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	8,792	6,291
Current Loans	813,286	147,724
Total Loans	822,078	154,015
Accruing Loans 90 Days or More Past Due	2,170	4,426
Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	8,927
Current Loans	1,150,883
Total Loans	1,159,810	1,315,005
Accruing Loans 90 Days or More Past Due	465
Oil and Gas [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current Loans	179,355
Total Loans	179,355	184,399
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		5,499
Current Loans		1,448,894
Total Loans		1,454,393
Accruing Loans 90 Days or More Past Due		354
Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		6,990
Current Loans		147,691
Total Loans		154,681
Accruing Loans 90 Days or More Past Due		555
Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		4,279
Current Loans		358,075
Total Loans		362,354
Accruing Loans 90 Days or More Past Due		491
Pegasus Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		851
Current Loans	554,548	429,854
Total Loans	554,548	430,705
Pegasus Bank [Member] | Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	272,000
30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	16,174	20,728
30 to 59 Days Past Due [Member] | Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,096
30 to 59 Days Past Due [Member] | Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	323
30 to 59 Days Past Due [Member] | Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	511
30 to 59 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	520
30 to 59 Days Past Due [Member] | Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,106
30 to 59 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,428
30 to 59 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		1,600
30 to 59 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		971
30 to 59 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		4,705
30 to 59 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		4,496
30 to 59 Days Past Due [Member] | Real Estate [Member] | Farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,297
30 to 59 Days Past Due [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,154
30 to 59 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	951	1,670
30 to 59 Days Past Due [Member] | Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,788
30 to 59 Days Past Due [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		2,290
30 to 59 Days Past Due [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		2,167
30 to 59 Days Past Due [Member] | Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		2,829
60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	5,574	6,656
60 to 89 Days Past Due [Member] | Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	108
60 to 89 Days Past Due [Member] | Real Estate [Member] | Construction and Development Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	86
60 to 89 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	55
60 to 89 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,463
60 to 89 Days Past Due [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		967
60 to 89 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		973
60 to 89 Days Past Due [Member] | Real Estate [Member] | Residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		1,028
60 to 89 Days Past Due [Member] | Real Estate [Member] | Farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	344
60 to 89 Days Past Due [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	501
60 to 89 Days Past Due [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,223	8
60 to 89 Days Past Due [Member] | Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,794
60 to 89 Days Past Due [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		1,446
60 to 89 Days Past Due [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		1,376
60 to 89 Days Past Due [Member] | Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		858
90 Days and Greater [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	23,784	22,409
90 Days and Greater [Member] | Real Estate [Member] | Commercial Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,164
90 Days and Greater [Member] | Real Estate [Member] | Commercial Real Estate Non-Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	34
90 Days and Greater [Member] | Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	1,606
90 Days and Greater [Member] | Real Estate [Member] | Construction Residential Real Estate Less than 60 Months [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	282
90 Days and Greater [Member] | Real Estate [Member] | Residential Real Estate First Lien [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	2,978
90 Days and Greater [Member] | Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		5,159
90 Days and Greater [Member] | Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		1,228
90 Days and Greater [Member] | Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		2,215
90 Days and Greater [Member] | Real Estate [Member] | Residential Real Estate Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		2,541
90 Days and Greater [Member] | Real Estate [Member] | Farmland [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,223
90 Days and Greater [Member] | Consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	534
90 Days and Greater [Member] | Other Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	6,618	4,613
90 Days and Greater [Member] | Commercial and Agricultural Non-Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans	$ 4,345
90 Days and Greater [Member] | Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		1,763
90 Days and Greater [Member] | Acquired Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		3,447
90 Days and Greater [Member] | Consumer Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		592
90 Days and Greater [Member] | Pegasus Bank [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due Loans		$ 851