Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 3,754,905	$ 4,364,123	$ 5,300,458	$ 4,556,125
Securities held to maturity	2,677,485	2,926,486
Loans, net	11,248,577	10,887,993
Deposits	24,324,397	24,135,930
Federal funds purchased and repurchase agreements	632,179	803,119
Junior subordinated deferrable interest debentures	137,115	137,115
Subordinated notes payable and other borrowings	100,000	100,000
Carrying Amount [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,754,905	4,364,123
Securities held to maturity	2,677,485	2,926,486
Cash surrender value of life insurance policies	174,514	172,050
Accrued interest receivable	97,434	128,436
Deposits	24,324,397	24,135,930
Federal funds purchased and repurchase agreements	632,179	803,119
Junior subordinated deferrable interest debentures	137,115	137,115
Subordinated notes payable and other borrowings	100,000	100,000
Accrued interest payable	1,127	1,132
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	11,248,577	10,887,993
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,754,905	4,364,123
Securities held to maturity	2,717,608	2,966,390
Cash surrender value of life insurance policies	174,514	172,050
Accrued interest receivable	97,434	128,436
Deposits	24,324,599	24,136,402
Federal funds purchased and repurchase agreements	632,179	803,119
Junior subordinated deferrable interest debentures	137,115	137,115
Subordinated notes payable and other borrowings	99,385	95,591
Accrued interest payable	1,127	1,132
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 11,280,538	$ 10,939,684

The sum of cash and cash equivalents plus Federal Funds Sold. Cash and cash equivalents consist of short term, highly liquid investments that are readily convertible to known amounts of cash and are so near their maturity that they present negligible risk of changes in value due to changes in interest rates -- usually with an original maturity less than 90 days.

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-Glossary Cash Equivalents

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Carrying amount as of the balance sheet date of amounts which could be received based on the terms of the insurance contract upon surrendering life policies owned by the entity.

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Amount after offset of short term borrowing where a bank borrows, at the federal funds rate, from another bank and securities that an entity sells and agrees to repurchase at a specified date for a specified price.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of interest payable on debt, including, but not limited to, trade payables.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of long-term debt (with maturities initially due after one year or beyond the operating cycle if longer) identified as Junior Subordinated Notes, which have a lower priority than senior instruments.

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Amount after allowance and 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 and loans held for sale.

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Including both current and noncurrent portions, carrying amount as of the balance sheet date of other forms of debt not elsewhere specified in the taxonomy with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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