Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	$ 13,473	$ 8,690
60 - 89 days past due	2,690	1,856
90 days and greater and still accruing	2,619	2,505
Nonaccrual	42,667	45,888
Total past due and nonaccrual	61,449	58,939
Current	4,395,859	4,224,894
Loans	4,457,308	4,283,833	4,204,704
Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	2,816	594
60 - 89 days past due	213	319
90 days and greater and still accruing	264	185
Nonaccrual	27,007	23,631
Total past due and nonaccrual	30,300	24,729
Current	1,021,809	996,327
Loans	1,052,109	1,021,056
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	0	0
60 - 89 days past due	1	0
90 days and greater and still accruing	0	0
Nonaccrual	236	2,567
Total past due and nonaccrual	237	2,567
Current	120,548	90,722
Loans	120,785	93,289
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	5,162	4,002
60 - 89 days past due	1,295	524
90 days and greater and still accruing	1,077	1,041
Nonaccrual	7,900	10,520
Total past due and nonaccrual	15,434	16,087
Current	1,210,910	1,246,631
Loans	1,226,344	1,262,718
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	1,797	1,199
60 - 89 days past due	122	23
90 days and greater and still accruing	0	13
Nonaccrual	7,306	8,966
Total past due and nonaccrual	9,225	10,201
Current	1,396,031	1,286,271
Loans	1,405,256	1,296,472
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30 - 59 days past due	3,698	2,895
60 - 89 days past due	1,059	990
90 days and greater and still accruing	1,278	1,266
Nonaccrual	218	204
Total past due and nonaccrual	6,253	5,355
Current	646,561	604,943
Loans	652,814	610,298
Commercial Financial Agricultural And Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,052,109	1,021,056	1,019,822
Real Estate Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	120,785	93,289	87,438
Residential real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,226,344	1,262,718	1,241,565
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,405,256	1,296,472	1,273,661
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 652,814	$ 610,298	$ 582,218

Financing receivables that are equal to or greater than 30 days past due.

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Financing receivables that are less than 60 days past due but more than 29 days past due.

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-Name Accounting Standards Codification

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Recorded investment in financing receivables that are 90 days or more past due and still accruing.

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Financing receivables that are current.

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Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

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