Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4,364,123,000	$ 4,556,125,000	$ 3,524,979,000
Securities held to maturity	2,926,486,000us-gaap_HeldToMaturitySecurities	3,139,748,000us-gaap_HeldToMaturitySecurities
Loans, net	10,887,993,000	9,423,262,000
Deposits	24,135,930,000us-gaap_Deposits	20,688,786,000us-gaap_Deposits
Federal funds purchased and repurchase agreements	803,119,000	668,253,000
Junior subordinated deferrable interest debentures	137,115,000us-gaap_JuniorSubordinatedNotes	123,712,000us-gaap_JuniorSubordinatedNotes
Subordinated notes payable and other borrowings	100,000,000us-gaap_OtherLongTermDebt	100,000,000us-gaap_OtherLongTermDebt
Carrying Amount [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4,364,123,000	4,556,125,000
Securities held to maturity	2,926,486,000	3,139,748,000
Cash surrender value of life insurance policies	172,050,000	141,108,000
Accrued interest receivable	128,436,000	99,281,000
Deposits	24,135,930,000	20,688,786,000
Federal funds purchased and repurchase agreements	803,119,000	668,253,000
Junior subordinated deferrable interest debentures	137,115,000	123,712,000
Subordinated notes payable and other borrowings	100,000,000	100,000,000
Accrued interest payable	1,132,000	1,300,000
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	10,887,993,000	9,423,262,000
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4,364,123,000	4,556,125,000
Securities held to maturity	2,966,390,000	3,029,663,000
Cash surrender value of life insurance policies	172,050,000	141,108,000
Accrued interest receivable	128,436,000	99,281,000
Deposits	24,136,402,000	20,689,323,000
Federal funds purchased and repurchase agreements	803,119,000	668,253,000
Junior subordinated deferrable interest debentures	137,115,000	123,712,000
Subordinated notes payable and other borrowings	95,591,000	92,552,000
Accrued interest payable	1,132,000	1,300,000
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 10,939,684,000	$ 9,582,734,000

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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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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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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