Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 54,225	$ 67,187	$ 61,234
Charge-offs	(17,472)	(35,178)	(16,969)
Recoveries	4,102	2,989	2,378
Provision for credit losses	11,196us-gaap_ProvisionForLoanAndLeaseLosses	19,227us-gaap_ProvisionForLoanAndLeaseLosses	20,544us-gaap_ProvisionForLoanAndLeaseLosses
Ending Balance	52,051	54,225	67,187
Ending balance: individually evaluated for impaired	9,461	8,824	17,778
Ending balance: collectively evaluated for impaired	42,590	45,401	49,409
Ending balance	4,457,308	4,283,833	4,204,704
Ending balance: individually evaluated for impaired	48,442	52,595	100,187
Ending balance: collectively evaluated for impaired	4,408,866	4,231,238	4,104,517
Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	22,663	19,852	18,200
Charge-offs	(8,911)	(18,399)	(5,207)
Recoveries	734	455	443
Provision for credit losses	15,141	20,755	6,416
Ending Balance	29,627	22,663	19,852
Ending balance: individually evaluated for impaired	9,304	7,364	10,331
Ending balance: collectively evaluated for impaired	20,323	15,299	9,521
Ending balance	1,052,109	1,021,056	1,019,822
Ending balance: individually evaluated for impaired	33,332	27,251	33,443
Ending balance: collectively evaluated for impaired	1,018,777	993,805	986,379
Real estate construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	6,600	8,928	6,756
Charge-offs	(296)	(773)	(3,601)
Recoveries	1,340	501	582
Provision for credit losses	(5,581)	(2,056)	5,191
Ending Balance	2,063	6,600	8,928
Ending balance: individually evaluated for impaired	0	94	300
Ending balance: collectively evaluated for impaired	2,063	6,506	8,628
Ending balance	120,785	93,289	87,438
Ending balance: individually evaluated for impaired	193	3,844	11,177
Ending balance: collectively evaluated for impaired	120,592	89,445	76,261
Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	7,727	5,908	8,237
Charge-offs	(3,153)	(1,814)	(3,828)
Recoveries	650	1,264	422
Provision for credit losses	(1,560)	2,369	1,077
Ending Balance	3,664	7,727	5,908
Ending balance: individually evaluated for impaired	56	1,282	780
Ending balance: collectively evaluated for impaired	3,608	6,445	5,128
Ending balance	1,226,344	1,262,718	1,241,565
Ending balance: individually evaluated for impaired	7,127	9,349	6,444
Ending balance: collectively evaluated for impaired	1,219,217	1,253,369	1,235,121
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,778	22,441	18,961
Charge-offs	(1,148)	(10,513)	(851)
Recoveries	612	136	410
Provision for credit losses	639	(286)	3,921
Ending Balance	11,881	11,778	22,441
Ending balance: individually evaluated for impaired	101	84	6,367
Ending balance: collectively evaluated for impaired	11,780	11,694	16,074
Ending balance	1,405,256	1,296,472	1,273,661
Ending balance: individually evaluated for impaired	7,790	12,151	49,123
Ending balance: collectively evaluated for impaired	1,397,466	1,284,321	1,224,538
Loans to individuals [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	5,457	4,132	4,244
Charge-offs	(3,964)	(3,679)	(3,482)
Recoveries	766	633	521
Provision for credit losses	2,557	4,371	2,849
Ending Balance	4,816	5,457	4,132
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	4,816	5,457	4,132
Ending balance	652,814	610,298	582,218
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	652,814	610,298	582,218
Unallocated [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance		5,926	4,836
Charge-offs		0	0
Recoveries		0	0
Provision for credit losses		(5,926)	1,090
Ending Balance		0	5,926
Ending balance: individually evaluated for impaired		0	0
Ending balance: collectively evaluated for impaired		$ 0	$ 5,926

A valuation allowance for financing receivables that are expected to be uncollectible.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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-Publisher FASB

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Amount of recovery of financing receivables doubtful of collection that were previously charged off.

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Amount of direct write-downs of financing receivables charged against the allowance.

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The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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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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Amount of expense related to estimated loss from loan and lease transactions.

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