Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 3,419,489	$ 5,053,047	$ 5,091,903	$ 4,141,445
Securities held to maturity	1,111,762	1,432,098
Loans, net	13,677,260	12,990,301
Deposits	26,348,854	26,872,389
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,227	136,184
Subordinated notes payable and other borrowings	98,669	98,552
Carrying Amount [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,419,489	5,053,047
Securities held to maturity	1,111,762	1,432,098
Cash surrender value of life insurance policies	182,603	180,477
Accrued interest receivable	137,553	167,508
Deposits	26,348,854	26,872,389
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	136,227	136,184
Subordinated notes payable and other borrowings	98,669	98,552
Accrued interest payable	4,904	3,358
Carrying Amount [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	13,677,260	12,990,301
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,419,489	5,053,047
Securities held to maturity	1,116,845	1,455,791
Cash surrender value of life insurance policies	182,603	180,477
Accrued interest receivable	137,553	167,508
Deposits	26,342,071	26,866,676
Federal funds purchased and repurchase agreements	1,083,777	1,147,824
Junior subordinated deferrable interest debentures, net of unamortized issuance costs	137,115	137,115
Subordinated notes payable and other borrowings	98,875	105,311
Accrued interest payable	4,904	3,358
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 13,621,005	$ 12,981,165

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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