Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 3,591,523	$ 4,364,123	$ 4,556,125	$ 3,524,979
Securities held to maturity	2,663,009	2,926,486
Loans, net	11,350,672	10,887,993
Deposits	24,343,595	24,135,930
Federal funds purchased and repurchase agreements	893,522	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	3,591,523	4,364,123
Securities held to maturity	2,663,009	2,926,486
Cash surrender value of life insurance policies	175,191	172,050
Accrued interest receivable	139,986	128,436
Deposits	24,343,595	24,135,930
Federal funds purchased and repurchase agreements	893,522	803,119
Junior subordinated deferrable interest debentures	137,115	137,115
Subordinated notes payable and other borrowings	100,000	100,000
Accrued interest payable	1,014	1,132
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	11,350,672	10,887,993
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,591,523	4,364,123
Securities held to maturity	2,710,685	2,966,390
Cash surrender value of life insurance policies	175,191	172,050
Accrued interest receivable	139,986	128,436
Deposits	24,344,007	24,136,402
Federal funds purchased and repurchase agreements	893,522	803,119
Junior subordinated deferrable interest debentures	137,115	137,115
Subordinated notes payable and other borrowings	99,000	95,591
Accrued interest payable	1,014	1,132
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 11,396,158	$ 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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Glossary Cash Equivalents
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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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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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