Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 7,706	$ 14,069
60 - 89 days past due	2,250	5,319
90 days and greater and still accruing	2,364	2,447
Nonaccrual	59,929	94,518
Total past due and nonaccrual	72,249	116,353
Current	4,167,755	4,088,351
Total	4,240,004	4,204,704	4,214,299
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	601	991
60 - 89 days past due	109	620
90 days and greater and still accruing	203	288
Nonaccrual	24,365	29,258
Total past due and nonaccrual	25,278	31,157
Current	983,038	988,665
Total	1,008,316	1,019,822
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	12	2
60 - 89 days past due	0	19
90 days and greater and still accruing	0	15
Nonaccrual	2,850	9,778
Total past due and nonaccrual	2,862	9,814
Current	71,163	77,624
Total	74,025	87,438
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	3,942	6,597
60 - 89 days past due	1,072	2,357
90 days and greater and still accruing	988	730
Nonaccrual	10,456	9,283
Total past due and nonaccrual	16,458	18,967
Current	1,264,532	1,222,598
Total	1,280,990	1,241,565
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	570	3,339
60 - 89 days past due	210	1,389
90 days and greater and still accruing	116	195
Nonaccrual	22,074	46,023
Total past due and nonaccrual	22,970	50,946
Current	1,249,396	1,222,715
Total	1,272,366	1,273,661
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,581	3,140
60 - 89 days past due	859	934
90 days and greater and still accruing	1,057	1,219
Nonaccrual	184	176
Total past due and nonaccrual	4,681	5,469
Current	599,626	576,749
Total	$ 604,307	$ 582,218