Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 5,640,106	$ 5,407,376	$ 5,374,782
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,130,638		1,199,800
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	259,825		249,255
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,518,850		1,416,926
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,172,615		1,963,001
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	558,178		$ 545,800
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	5,221	2,570
Financing Receivable, Recorded Investment, Current	544,828	397,445
Total	550,049	400,015
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	244	449
Financing Receivable, Recorded Investment, Current	42,319	40,193
Total	42,563	40,642
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, Recorded Investment, Current	9,130	5,963
Total	9,130	5,963
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	2,857	945
Financing Receivable, Recorded Investment, Current	264,640	219,306
Total	267,497	220,251
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	2,030	1,077
Financing Receivable, Recorded Investment, Current	222,298	125,834
Total	224,328	126,911
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	90	99
Financing Receivable, Recorded Investment, Current	6,441	6,149
Total	6,531	6,248
Acquired Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	426	190
Acquired Loans [Member] | Financing Receivables, 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	160	6
Acquired Loans [Member] | Financing Receivables, 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] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	238	148
Acquired Loans [Member] | Financing Receivables, 30 to 59 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] | Financing Receivables, 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	36
Acquired Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	37	36
Acquired Loans [Member] | Financing Receivables, 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	7
Acquired Loans [Member] | Financing Receivables, 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] | Financing Receivables, 60 to 89 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	9	9
Acquired Loans [Member] | Financing Receivables, 60 to 89 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] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	28	20
Acquired Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	50	109
Acquired Loans [Member] | Financing Receivables, Equal to 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	11	0
Acquired Loans [Member] | Financing Receivables, Equal to 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] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	21	83
Acquired Loans [Member] | Financing Receivables, Equal to 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] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	18	26
Acquired Loans [Member] | Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	4,708	2,235
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	73	436
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	2,589	705
Acquired Loans [Member] | Non Accrual Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	2,030	1,077
Acquired Loans [Member] | Non Accrual Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	16	17
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	40,238	41,362
Financing Receivable, Recorded Investment, Current	5,049,819	4,965,999
Total	5,090,057	5,007,361
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	20,232	19,220
Financing Receivable, Recorded Investment, Current	1,067,843	1,103,521
Total	1,088,075	1,122,741
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	199
Financing Receivable, Recorded Investment, Current	250,695	242,706
Total	250,695	242,905
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	11,065	12,944
Financing Receivable, Recorded Investment, Current	1,240,288	1,193,175
Total	1,251,353	1,206,119
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	6,159	5,472
Financing Receivable, Recorded Investment, Current	1,942,128	1,886,713
Total	1,948,287	1,892,185
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	2,782	3,527
Financing Receivable, Recorded Investment, Current	548,865	539,884
Total	551,647	543,411
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,408	9,292
Originated Loans [Member] | Financing Receivables, 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	53	378
Originated Loans [Member] | Financing Receivables, 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	199
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,192	4,618
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	448	2,198
Originated Loans [Member] | Financing Receivables, 30 to 59 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,715	1,899
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,162	1,883
Originated Loans [Member] | Financing Receivables, 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	317	61
Originated Loans [Member] | Financing Receivables, 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] | Financing Receivables, 60 to 89 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	830	1,025
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,727	28
Originated Loans [Member] | Financing Receivables, 60 to 89 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	288	769
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,675	1,745
Originated Loans [Member] | Financing Receivables, Equal to 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	91	40
Originated Loans [Member] | Financing Receivables, Equal to 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] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	800	1,076
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	160	6
Originated Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	624	623
Originated Loans [Member] | Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	29,993	28,442
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	19,771	18,741
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	6,243	6,225
Originated Loans [Member] | Non Accrual Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,824	3,240
Originated Loans [Member] | Non Accrual Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	$ 155	$ 236