Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 4,240,004	$ 4,204,704	$ 4,214,299
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	4,059,411	3,916,202
OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	73,397	120,205
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	107,196	164,775
Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3,522
Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	180,593	288,502
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,008,316	1,019,822
Commercial, financial, agricultural and other [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	940,572	925,868
Commercial, financial, agricultural and other [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	25,287	31,049
Commercial, financial, agricultural and other [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	42,457	62,905
Commercial, financial, agricultural and other [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial, financial, agricultural and other [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	67,744	93,954
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	74,025	87,438
Real estate construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	60,322	64,353
Real estate construction [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,046	925
Real estate construction [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	12,657	18,638
Real estate construction [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	3,522
Real estate construction [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	13,703	23,085
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,280,990	1,241,565
Residential real estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,263,936	1,224,849
Residential real estate [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	5,375	5,647
Residential real estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	11,679	11,069
Residential real estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Residential real estate [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	17,054	16,716
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,272,366	1,273,661
Commercial Real Estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,190,752	1,119,093
Commercial Real Estate [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	41,688	82,581
Commercial Real Estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	39,926	71,987
Commercial Real Estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Commercial Real Estate [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	81,614	154,568
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	604,307	582,218
Loans to individuals [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	603,829	582,039
Loans to individuals [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1	3
Loans to individuals [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	477	176
Loans to individuals [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0	0
Loans to individuals [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 478	$ 179