Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 6,003,059	$ 5,774,139	$ 5,640,106
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,236,424		1,130,638
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	441,854		259,825
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,579,441		1,518,850
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,118,582		2,172,615
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	626,758		$ 558,178
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	7,655	3,512
Financing Receivable, Not Past Due	445,005	495,331
Total	452,660	498,843
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	4,718	205
Financing Receivable, Not Past Due	30,001	37,321
Total	34,719	37,526
Acquired Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	0	0
Financing Receivable, Not Past Due	6,469	5,970
Total	6,469	5,970
Acquired Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	2,279	2,181
Financing Receivable, Not Past Due	226,397	246,579
Total	228,676	248,760
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	589	1,042
Financing Receivable, Not Past Due	178,385	200,153
Total	178,974	201,195
Acquired Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	69	84
Financing Receivable, Not Past Due	3,753	5,308
Total	3,822	5,392
Acquired Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	456	273
Acquired Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	1
Acquired Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	295	226
Acquired Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	133	0
Acquired Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	28	46
Acquired Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	111	36
Acquired Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	59	24
Acquired Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	46	0
Acquired Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	6	12
Acquired Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	165	38
Acquired Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	143	27
Acquired Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	22	11
Acquired Loans [Member] | Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	6,923	3,165
Acquired Loans [Member] | Non Accrual Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	4,718	204
Acquired Loans [Member] | Non Accrual Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Non Accrual Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,782	1,904
Acquired Loans [Member] | Non Accrual Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	410	1,042
Acquired Loans [Member] | Non Accrual Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	13	15
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	31,610	30,100
Financing Receivable, Not Past Due	5,518,789	5,245,196
Total	5,550,399	5,275,296
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	6,158	10,692
Financing Receivable, Not Past Due	1,195,547	1,090,255
Total	1,201,705	1,100,947
Originated Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	0	212
Financing Receivable, Not Past Due	435,385	352,796
Total	435,385	353,008
Originated Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	11,242	11,435
Financing Receivable, Not Past Due	1,339,523	1,302,210
Total	1,350,765	1,313,645
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	9,783	3,998
Financing Receivable, Not Past Due	1,929,825	1,918,351
Total	1,939,608	1,922,349
Originated Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	4,427	3,763
Financing Receivable, Not Past Due	618,509	581,584
Total	622,936	585,347
Originated Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	6,750	6,893
Originated Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	485	130
Originated Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	212
Originated Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,874	3,697
Originated Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	734	492
Originated Loans [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,657	2,362
Originated Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,713	1,558
Originated Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	39	247
Originated Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Originated Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,077	710
Originated Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	717	69
Originated Loans [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	880	532
Originated Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,491	1,544
Originated Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	79	92
Originated Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Originated Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,590	790
Originated Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	191	0
Originated Loans [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	631	662
Originated Loans [Member] | Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	19,656	20,105
Originated Loans [Member] | Non Accrual Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,555	10,223
Originated Loans [Member] | Non Accrual Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Originated Loans [Member] | Non Accrual Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,701	6,238
Originated Loans [Member] | Non Accrual Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	8,141	3,437
Originated Loans [Member] | Non Accrual Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	$ 259	$ 207