Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 22,010	$ 14,069
60 - 89 days past due	6,312	5,319
90 days and greater and still accruing	3,927	2,447
Nonaccrual	64,141	94,518
Total past due and nonaccrual	96,390	116,353
Current	4,122,420	4,088,351
Total	4,218,810	4,204,704	4,128,588
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	3,054	991
60 - 89 days past due	2,380	620
90 days and greater and still accruing	1,108	288
Nonaccrual	28,666	29,258
Total past due and nonaccrual	35,208	31,157
Current	1,022,455	988,665
Total	1,057,663	1,019,822
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	206	2
60 - 89 days past due	15	19
90 days and greater and still accruing	500	15
Nonaccrual	7,112	9,778
Total past due and nonaccrual	7,833	9,814
Current	62,628	77,624
Total	70,461	87,438
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	6,282	6,597
60 - 89 days past due	2,420	2,357
90 days and greater and still accruing	963	730
Nonaccrual	9,740	9,283
Total past due and nonaccrual	19,405	18,967
Current	1,236,110	1,222,598
Total	1,255,515	1,241,565
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	9,903	3,339
60 - 89 days past due	921	1,389
90 days and greater and still accruing	279	195
Nonaccrual	18,456	46,023
Total past due and nonaccrual	29,559	50,946
Current	1,214,117	1,222,715
Total	1,243,676	1,273,661
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,565	3,140
60 - 89 days past due	576	934
90 days and greater and still accruing	1,077	1,219
Nonaccrual	167	176
Total past due and nonaccrual	4,385	5,469
Current	587,110	576,749
Total	$ 591,495	$ 582,218