Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 18,652	$ 19,200
60 - 89 days past due	3,654	5,307
90 days and greater and still accruing	2,998	11,015
Nonaccrual	86,730	78,476
Total past due and nonaccrual	112,034	113,998
Current	4,102,265	3,929,645
Loans	4,214,299	4,043,643	3,973,723
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	1,440	5,433
60 - 89 days past due	679	824
90 days and greater and still accruing	221	287
Nonaccrual	28,827	33,459
Total past due and nonaccrual	31,167	40,003
Current	1,055,852	956,736
Loans	1,087,019	996,739
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	235	0
60 - 89 days past due	0	180
90 days and greater and still accruing	152	0
Nonaccrual	11,569	14,911
Total past due and nonaccrual	11,956	15,091
Current	83,469	61,473
Loans	95,425	76,564
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	7,642	7,144
60 - 89 days past due	1,986	2,100
90 days and greater and still accruing	1,277	8,767
Nonaccrual	8,910	3,153
Total past due and nonaccrual	19,815	21,164
Current	1,208,513	1,115,895
Loans	1,228,328	1,137,059
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	6,651	3,671
60 - 89 days past due	169	1,241
90 days and greater and still accruing	162	157
Nonaccrual	37,340	26,953
Total past due and nonaccrual	44,322	32,022
Current	1,172,927	1,235,410
Loans	1,217,249	1,267,432
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,684	2,952
60 - 89 days past due	820	962
90 days and greater and still accruing	1,186	1,804
Nonaccrual	84	0
Total past due and nonaccrual	4,774	5,718
Current	581,504	560,131
Loans	$ 586,278	$ 565,849