Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 12,797	$ 8,690
60 - 89 days past due	2,981	1,856
90 days and greater and still accruing	2,374	2,505
Nonaccrual	34,093	45,888
Total past due and nonaccrual	52,245	58,939
Current	4,359,236	4,224,894
Total	4,411,481	4,283,833	4,240,004
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	467	594
60 - 89 days past due	165	319
90 days and greater and still accruing	267	185
Nonaccrual	18,847	23,631
Total past due and nonaccrual	19,746	24,729
Current	1,051,785	996,327
Total	1,071,531	1,021,056
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	0	0
60 - 89 days past due	0	0
90 days and greater and still accruing	0	0
Nonaccrual	291	2,567
Total past due and nonaccrual	291	2,567
Current	115,497	90,722
Total	115,788	93,289
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	6,371	4,002
60 - 89 days past due	1,636	524
90 days and greater and still accruing	778	1,041
Nonaccrual	8,286	10,520
Total past due and nonaccrual	17,071	16,087
Current	1,217,771	1,246,631
Total	1,234,842	1,262,718
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,162	1,199
60 - 89 days past due	75	23
90 days and greater and still accruing	0	13
Nonaccrual	6,408	8,966
Total past due and nonaccrual	8,645	10,201
Current	1,336,657	1,286,271
Total	1,345,302	1,296,472
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	3,797	2,895
60 - 89 days past due	1,105	990
90 days and greater and still accruing	1,329	1,266
Nonaccrual	261	204
Total past due and nonaccrual	6,492	5,355
Current	637,526	604,943
Total	$ 644,018	$ 610,298