Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 4,411,481	$ 4,283,833
Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	4,272,032	4,121,472
OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	75,725	79,124
Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	59,804	83,237
Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	3,920	0
Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	139,449	162,361
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,071,531	1,021,056
Commercial, financial, agricultural and other [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	996,609	943,107
Commercial, financial, agricultural and other [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	40,620	35,429
Commercial, financial, agricultural and other [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	30,382	42,520
Commercial, financial, agricultural and other [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	3,920	0
Commercial, financial, agricultural and other [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	74,922	77,949
Real estate construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	115,788	93,289
Real estate construction [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	106,989	79,679
Real estate construction [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	8,222	9,710
Real estate construction [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	577	3,900
Real estate construction [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Real estate construction [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	8,799	13,610
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,234,842	1,262,718
Residential real estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,223,231	1,245,422
Residential real estate [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,874	5,161
Residential real estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	9,737	12,135
Residential real estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Residential real estate [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	11,611	17,296
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,345,302	1,296,472
Commercial Real Estate [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,301,447	1,243,170
Commercial Real Estate [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	25,009	28,823
Commercial Real Estate [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	18,846	24,479
Commercial Real Estate [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Commercial Real Estate [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	43,855	53,302
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	644,018	610,298
Loans to individuals [Member] | Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	643,756	610,094
Loans to individuals [Member] | OAEM [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	1
Loans to individuals [Member] | Substandard [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	262	203
Loans to individuals [Member] | Doubtful [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	0	0
Loans to individuals [Member] | Total Non-Pass [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 262	$ 204