Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available for sale securities	$ 2,790,468	$ 2,691,037
Held to maturity securities	2,935,267	2,979,547
Derivative financial instruments	153	3,325
Derivative financial instruments	10,508	2,160
Total Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	516,214	494,466
Available for sale securities	2,790,468	2,691,037
Held to maturity securities	2,979,401	2,935,856
Loans, net	20,062,456	19,726,887
Loans held for sale	36,150	28,150
Derivative financial instruments	51,609	16,980
Deposits	23,220,712	23,129,574
Federal funds purchased	550	425
Securities sold under agreements to repurchase	631,048	428,599
FHLB short-term borrowings	1,010,000	1,160,104
Long-term debt	224,547	223,135
Derivative financial instruments	19,248	22,227
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	516,214	494,466
Federal funds purchased	550	425
Securities sold under agreements to repurchase	631,048	428,599
FHLB short-term borrowings	1,010,000	1,160,104
Total Fair Value [Member] \| Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available for sale securities	2,790,468	2,691,037
Held to maturity securities	2,979,401	2,935,856
Loans, net	168,190	170,918
Loans held for sale	36,150	28,150
Derivative financial instruments	51,609	16,980
Long-term debt	224,547	223,135
Derivative financial instruments	12,646	14,923
Total Fair Value [Member] \| Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Loans, net	19,894,266	19,555,969
Deposits	23,220,712	23,129,574
Derivative financial instruments	6,602	7,304
Carrying Amount [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	516,214	494,466
Available for sale securities	2,790,468	2,691,037
Held to maturity securities	2,935,267	2,979,547
Loans, net	19,980,187	19,831,897
Loans held for sale	36,150	28,150
Derivative financial instruments	51,609	16,980
Deposits	23,236,042	23,150,185
Federal funds purchased	550	425
Securities sold under agreements to repurchase	631,048	428,599
FHLB short-term borrowings	1,010,000	1,160,104
Long-term debt	232,754	224,993
Derivative financial instruments	$ 19,248	$ 22,227

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, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset.

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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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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 of investment in debt security measured at amortized cost (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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