Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 52,051	$ 54,225
Charge-offs	(7,109)	(3,646)
Recoveries	596	696
Provision for Loan, Lease, and Other Losses	1,159	3,231
Ending Balance	46,697	54,506
Ending balance: individually evaluated for impaired	3,829	10,503
Ending balance: collectively evaluated for impaired	42,868	44,003
Total	4,437,601	4,252,213	4,457,308
Ending balance: individually evaluated for impaired	39,659	49,403
Ending balance: collectively evaluated for impaired	4,397,942	4,202,810
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	29,627	22,663
Charge-offs	(5,080)	(1,601)
Recoveries	200	85
Provision for Loan, Lease, and Other Losses	(341)	4,978
Ending Balance	24,406	26,125
Ending balance: individually evaluated for impaired	3,397	9,886
Ending balance: collectively evaluated for impaired	21,009	16,239
Total	1,066,788	1,017,412	1,052,109
Ending balance: individually evaluated for impaired	24,586	31,730
Ending balance: collectively evaluated for impaired	1,042,202	985,682
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,063	6,600
Charge-offs	0	0
Recoveries	0	169
Provision for Loan, Lease, and Other Losses	(535)	(555)
Ending Balance	1,528	6,214
Ending balance: individually evaluated for impaired	0	74
Ending balance: collectively evaluated for impaired	1,528	6,140
Total	107,882	95,110	120,785
Ending balance: individually evaluated for impaired	199	2,797
Ending balance: collectively evaluated for impaired	107,683	92,313
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,664	7,727
Charge-offs	(566)	(1,095)
Recoveries	96	244
Provision for Loan, Lease, and Other Losses	193	(850)
Ending Balance	3,387	6,026
Ending balance: individually evaluated for impaired	165	496
Ending balance: collectively evaluated for impaired	3,222	5,530
Total	1,210,511	1,240,169	1,226,344
Ending balance: individually evaluated for impaired	7,071	9,103
Ending balance: collectively evaluated for impaired	1,203,440	1,231,066
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	11,881	11,778
Charge-offs	(202)	(140)
Recoveries	138	20
Provision for Loan, Lease, and Other Losses	670	(539)
Ending Balance	12,487	11,119
Ending balance: individually evaluated for impaired	267	47
Ending balance: collectively evaluated for impaired	12,220	11,072
Total	1,400,276	1,290,852	1,405,256
Ending balance: individually evaluated for impaired	7,803	5,773
Ending balance: collectively evaluated for impaired	1,392,473	1,285,079
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,816	5,457
Charge-offs	(1,261)	(810)
Recoveries	162	178
Provision for Loan, Lease, and Other Losses	1,172	197
Ending Balance	4,889	5,022
Ending balance: individually evaluated for impaired	0	0
Ending balance: collectively evaluated for impaired	4,889	5,022
Total	652,144	608,670	652,814
Ending balance: individually evaluated for impaired	0	0
Ending balance: collectively evaluated for impaired	$ 652,144	$ 608,670

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

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

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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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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 loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

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