Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available for sale securities	$ 6,630,648	$ 5,999,327
Held to maturity securities	1,375,342	1,357,170
Derivative financial instruments	9,874	2,869
Derivative financial instruments	24,307	44,841
Total Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	2,847,784	1,860,092
Available for sale securities	6,630,648	5,999,327
Held to maturity securities	1,455,224	1,467,581
Loans, net	21,282,994	21,533,384
Loans held for sale	124,677	136,063
Derivative financial instruments	99,726	150,180
Deposits	29,185,386	27,679,321
Federal funds purchased	4,300	300
Securities sold under agreements to repurchase	548,447	567,213
FHLB short-term borrowings	1,106,829	1,147,335
Long-term debt	394,806	404,880
Derivative financial instruments	42,509	55,257
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	2,847,784	1,860,092
Federal funds purchased	4,300	300
Securities sold under agreements to repurchase	548,447	567,213
Total Fair Value [Member] \| Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available for sale securities	6,630,648	5,999,327
Held to maturity securities	1,455,224	1,467,581
Loans, net	33,859	60,451
Loans held for sale	124,677	136,063
Derivative financial instruments	99,726	150,180
FHLB short-term borrowings	1,106,829	1,147,335
Long-term debt	394,806	404,880
Derivative financial instruments	37,252	49,612
Total Fair Value [Member] \| Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Loans, net	21,249,135	21,472,933
Deposits	29,185,386	27,679,321
Derivative financial instruments	5,257	5,645
Carrying Amount [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	2,847,784	1,860,092
Available for sale securities	6,630,648	5,999,327
Held to maturity securities	1,375,342	1,357,170
Loans, net	21,240,499	21,339,754
Loans held for sale	124,677	136,063
Derivative financial instruments	99,726	150,180
Deposits	29,210,520	27,697,877
Federal funds purchased	4,300	300
Securities sold under agreements to repurchase	548,447	567,213
FHLB short-term borrowings	1,100,000	1,100,000
Long-term debt	397,583	378,322
Derivative financial instruments	$ 42,509	$ 55,257

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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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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Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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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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