Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 2,281,454	$ 3,955,779	$ 4,680,948	$ 5,053,047
Securities held to maturity	1,077,279	1,106,057
Loans, net	14,269,989	13,967,601
Deposits	26,294,863	27,149,204
Federal funds purchased and repurchase agreements	1,227,149	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,256	136,242
Subordinated notes payable and other borrowings	98,747	98,708
Carrying Amount [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,281,454	3,955,779
Securities held to maturity	1,077,279	1,106,057
Cash surrender value of life insurance policies	184,371	183,473
Accrued interest receivable	142,977	188,989
Deposits	26,294,863	27,149,204
Federal funds purchased and repurchase agreements	1,227,149	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,256	136,242
Subordinated notes payable and other borrowings	98,747	98,708
Accrued interest payable	7,844	7,394
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	14,269,989	13,967,601
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,281,454	3,955,779
Securities held to maturity	1,097,858	1,116,953
Cash surrender value of life insurance policies	184,371	183,473
Accrued interest receivable	142,977	188,989
Deposits	26,291,609	27,143,572
Federal funds purchased and repurchase agreements	1,227,149	1,367,548
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes payable and other borrowings	101,238	98,458
Accrued interest payable	7,844	7,394
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 14,256,028	$ 13,933,239

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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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 of investment in debt security measured at amortized cost (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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