Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 5,375,847	$ 4,879,347	$ 4,860,652
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,154,225		1,207,447
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	259,129		229,375
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,423,422		1,185,759
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,990,264		1,683,015
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	548,807		$ 555,056
Acquired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	5,513	1,302
Financing Receivable, Recorded Investment, Current	419,485	99,574
Total	424,998	100,876
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	1,163	486
Financing Receivable, Recorded Investment, Current	41,730	7,913
Total	42,893	8,399
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	10,222	1,781
Total	10,222	1,781
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	1,049	643
Financing Receivable, Recorded Investment, Current	231,511	62,698
Total	232,560	63,341
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	3,199	162
Financing Receivable, Recorded Investment, Current	129,502	25,005
Total	132,701	25,167
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	102	11
Financing Receivable, Recorded Investment, Current	6,520	2,177
Total	6,622	2,188
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	375	635
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	157	486
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	175	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	43	1
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	88	46
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	0
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	83	39
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	5	7
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	37	34
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	0	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	0	34
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	37	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	5,013	587
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	1,006	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	791	422
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,199	162
Acquired Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	17	3
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Recorded Investment Equal To Or Greater Than Thirty Days Past Due	30,750	40,497
Financing Receivable, Recorded Investment, Current	4,920,099	4,737,974
Total	4,950,849	4,778,471
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	11,539	20,554
Financing Receivable, Recorded Investment, Current	1,099,793	1,110,594
Total	1,111,332	1,131,148
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	183
Financing Receivable, Recorded Investment, Current	248,907	217,657
Total	248,907	217,840
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	10,928	12,009
Financing Receivable, Recorded Investment, Current	1,179,934	1,153,842
Total	1,190,862	1,165,851
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	4,861	4,092
Financing Receivable, Recorded Investment, Current	1,852,702	1,712,951
Total	1,857,563	1,717,043
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	3,422	3,659
Financing Receivable, Recorded Investment, Current	538,763	542,930
Total	542,185	546,589
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	6,926	8,601
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	239	2,380
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	183
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	3,165	4,133
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	1,444	265
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	2,078	1,640
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	1,191	2,363
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	8	171
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	734	1,089
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	0	327
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	449	776
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	1,295	2,097
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	75	75
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	564	995
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	0	57
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	656	970
Originated Loans [Member] | Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	21,338	27,436
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	11,217	17,928
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	0	0
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	6,465	5,792
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	3,417	3,443
Originated Loans [Member] | Nonaccrual Commercial Loans [Member] | Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total past due and nonaccrual	$ 239	$ 273