Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 8,845	$ 8,690
60 - 89 days past due	6,654	1,856
90 days and greater and still accruing	2,410	2,505
Nonaccrual	35,721	45,888
Total past due and nonaccrual	53,630	58,939
Current	4,280,584	4,224,894
Total	4,334,214	4,283,833	4,229,752
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	687	594
60 - 89 days past due	173	319
90 days and greater and still accruing	230	185
Nonaccrual	19,678	23,631
Total past due and nonaccrual	20,768	24,729
Current	1,041,233	996,327
Total	1,062,001	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	303	2,567
Total past due and nonaccrual	303	2,567
Current	100,406	90,722
Total	100,709	93,289
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	4,649	4,002
60 - 89 days past due	1,222	524
90 days and greater and still accruing	891	1,041
Nonaccrual	8,734	10,520
Total past due and nonaccrual	15,496	16,087
Current	1,223,295	1,246,631
Total	1,238,791	1,262,718
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	956	1,199
60 - 89 days past due	4,163	23
90 days and greater and still accruing	0	13
Nonaccrual	6,764	8,966
Total past due and nonaccrual	11,883	10,201
Current	1,294,869	1,286,271
Total	1,306,752	1,296,472
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,553	2,895
60 - 89 days past due	1,096	990
90 days and greater and still accruing	1,289	1,266
Nonaccrual	242	204
Total past due and nonaccrual	5,180	5,355
Current	620,781	604,943
Total	$ 625,961	$ 610,298