Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 8,690	$ 14,069
60 - 89 days past due	1,856	5,319
90 days and greater and still accruing	2,505	2,447
Nonaccrual	45,888	94,518
Total past due and nonaccrual	58,939	116,353
Current	4,224,894	4,088,351
Loans	4,283,833	4,204,704	4,043,643
Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	594	991
60 - 89 days past due	319	620
90 days and greater and still accruing	185	288
Nonaccrual	23,631	29,258
Total past due and nonaccrual	24,729	31,157
Current	996,327	988,665
Loans	1,021,056	1,019,822
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	0	2
60 - 89 days past due	0	19
90 days and greater and still accruing	0	15
Nonaccrual	2,567	9,778
Total past due and nonaccrual	2,567	9,814
Current	90,722	77,624
Loans	93,289	87,438
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	4,002	6,597
60 - 89 days past due	524	2,357
90 days and greater and still accruing	1,041	730
Nonaccrual	10,520	9,283
Total past due and nonaccrual	16,087	18,967
Current	1,246,631	1,222,598
Loans	1,262,718	1,241,565
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	1,199	3,339
60 - 89 days past due	23	1,389
90 days and greater and still accruing	13	195
Nonaccrual	8,966	46,023
Total past due and nonaccrual	10,201	50,946
Current	1,286,271	1,222,715
Loans	1,296,472	1,273,661
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,895	3,140
60 - 89 days past due	990	934
90 days and greater and still accruing	1,266	1,219
Nonaccrual	204	176
Total past due and nonaccrual	5,355	5,469
Current	604,943	576,749
Loans	610,298	582,218
Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,021,056	1,019,822	996,739
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	93,289	87,438	76,564
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,262,718	1,241,565	1,137,059
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,296,472	1,273,661	1,267,432
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 610,298	$ 582,218	$ 565,849