Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,218,810	$ 4,204,704	$ 4,128,588
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,963,944	3,916,202
OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	109,791	120,205
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	141,553	164,775
Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,522	3,522
Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	254,866	288,502
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,057,663	1,019,822
Commercial, financial, agricultural and other [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	942,673	925,868
Commercial, financial, agricultural and other [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	43,909	31,049
Commercial, financial, agricultural and other [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	71,081	62,905
Commercial, financial, agricultural and other [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0	0
Commercial, financial, agricultural and other [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	114,990	93,954
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	70,461	87,438
Real estate construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	50,664	64,353
Real estate construction [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	911	925
Real estate construction [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	15,364	18,638
Real estate construction [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	3,522	3,522
Real estate construction [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	19,797	23,085
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,255,515	1,241,565
Residential real estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,238,633	1,224,849
Residential real estate [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	5,075	5,647
Residential real estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	11,807	11,069
Residential real estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0	0
Residential real estate [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	16,882	16,716
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,243,676	1,273,661
Commercial real estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,140,648	1,119,093
Commercial real estate [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	59,894	82,581
Commercial real estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	43,134	71,987
Commercial real estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0	0
Commercial real estate [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	103,028	154,568
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	591,495	582,218
Loans to individuals [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	591,326	582,039
Loans to individuals [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2	3
Loans to individuals [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	167	176
Loans to individuals [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	0	0
Loans to individuals [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 169	$ 179