Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Contract	Sep. 30, 2011 Contract	Sep. 30, 2012 Contract	Sep. 30, 2011 Contract
Financing Receivable, Modifications [Line Items]
Number of Contracts	7	6	15	37
Extend Maturity	$ 1,153	$ 39	$ 2,422	$ 17,656
Modify Rate	685	73	800	404
Modify Payments	1,509	635	7,621	4,375
Total Pre-Modification Outstanding Recorded Investment	3,347	747	10,843	22,435
Post-Modification Outstanding Recorded Investment	3,533	738	10,336	22,419
Specific Reserve	844	7	3,238	2,164
Commercial, financial, agricultural and other [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	6	1	10	10
Extend Maturity	1,153	0	1,599	100
Modify Rate	169	0	187	105
Modify Payments	1,509	50	7,538	2,218
Total Pre-Modification Outstanding Recorded Investment	2,831	50	9,324	2,423
Post-Modification Outstanding Recorded Investment	3,004	46	8,885	2,402
Specific Reserve	746	0	3,140	606
Real estate construction [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	0	0	1	4
Extend Maturity	0	0	823	354
Modify Rate	0	0	0	0
Modify Payments	0	0	0	0
Total Pre-Modification Outstanding Recorded Investment	0	0	823	354
Post-Modification Outstanding Recorded Investment	0	0	791	506
Specific Reserve	0	0	0	0
Residential real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	0	2	3	5
Extend Maturity	0	0	0	0
Modify Rate	0	73	97	100
Modify Payments	0	104	83	179
Total Pre-Modification Outstanding Recorded Investment	0	177	180	279
Post-Modification Outstanding Recorded Investment	0	175	131	274
Specific Reserve	0	7	0	7
Commercial real estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	1	3	1	18
Extend Maturity	0	39	0	17,202
Modify Rate	516	0	516	199
Modify Payments	0	481	0	1,978
Total Pre-Modification Outstanding Recorded Investment	516	520	516	19,379
Post-Modification Outstanding Recorded Investment	529	517	529	19,237
Specific Reserve	$ 98	$ 0	$ 98	$ 1,551