Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 5,053,047	$ 4,141,445	$ 3,591,523	$ 4,364,123
Securities held to maturity	1,432,098	2,250,460
Loans, net	12,990,301	11,822,347
Deposits	26,872,389	25,811,575
Federal funds purchased and repurchase agreements	1,147,824	976,992
Junior subordinated deferrable interest debentures	136,184	136,127
Subordinated notes payable and other borrowings	98,552	99,990
Carrying Amount [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5,053,047	4,141,445
Securities held to maturity	1,432,098	2,250,460
Cash surrender value of life insurance policies	180,477	177,884
Accrued interest receivable	167,508	156,714
Deposits	26,872,389	25,811,575
Federal funds purchased and repurchase agreements	1,147,824	976,992
Junior subordinated deferrable interest debentures	136,184	136,127
Subordinated notes payable and other borrowings	98,552	99,990
Accrued interest payable	3,358	1,204
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	12,990,301	11,822,347
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5,053,047	4,141,445
Securities held to maturity	1,455,791	2,262,747
Cash surrender value of life insurance policies	180,477	177,884
Accrued interest receivable	167,508	156,714
Deposits	26,866,676	25,812,039
Federal funds purchased and repurchase agreements	1,147,824	976,992
Junior subordinated deferrable interest debentures	137,115	137,115
Subordinated notes payable and other borrowings	105,311	100,000
Accrued interest payable	3,358	1,204
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 12,981,165	$ 11,903,956

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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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 the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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