Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	$ 2,918,185	$ 2,910,869
Held to maturity securities	3,069,262	2,977,511
Derivative financial instruments	6,751	2,148
Derivative financial instruments	309	3,938
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	447,683
Federal funds purchased	50,325
Securities sold under agreements to repurchase	415,960
FHLB Short-term borrowings	1,810,362
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities	2,918,185
Held to maturity securities	2,975,455
Loans, net	180,730
Loans held for sale	29,043
Derivative financial instruments	11,411
Long-term debt	213,076
Derivative financial instruments	28,791
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	18,843,738
Deposits	22,373,052
Total Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	447,683	479,332
Available for sale securities	2,918,185	2,910,869
Held to maturity securities	2,975,455	2,962,010
Loans, net	19,024,468	18,587,508
Loans held for sale	29,043	39,865
Derivative financial instruments	11,411	14,157
Deposits	22,373,052	22,238,847
Federal funds purchased	50,325	140,754
Securities sold under agreements to repurchase	415,960	430,569
FHLB Short-term borrowings	1,810,362	1,132,567
Long-term debt	213,076	303,631
Derivative financial instruments	28,791	14,389
Total Fair Value [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold		479,332
Federal funds purchased		140,754
Securities sold under agreements to repurchase		430,569
FHLB Short-term borrowings		1,132,567
Total Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale securities		2,910,869
Held to maturity securities		2,962,010
Loans, net		184,205
Loans held for sale		39,865
Derivative financial instruments		14,157
Long-term debt		303,631
Derivative financial instruments		14,389
Total Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net		18,403,303
Deposits		22,238,847
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	447,683	479,332
Available for sale securities	2,918,185	2,910,869
Held to maturity securities	3,069,262	2,977,511
Loans, net	19,329,167	18,786,855
Loans held for sale	29,043	39,865
Derivative financial instruments	11,411	14,157
Deposits	22,417,807	22,253,202
Federal funds purchased	50,325	140,754
Securities sold under agreements to repurchase	415,960	430,569
FHLB Short-term borrowings	1,810,362	1,132,567
Long-term debt	215,912	305,513
Derivative financial instruments	$ 28,791	$ 14,389

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value portion of short term borrowing where a bank borrows, at the federal funds rate, from another bank.

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Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).

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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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Fair value portion of loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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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. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Fair value portion of securities that an entity sells and agrees to repurchase at a specified date for a specified price.

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