Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available for sale securities	$ 4,893,996	$ 4,675,304	$ 4,675,304
Held to maturity securities	1,480,494	1,568,009
Derivative financial instruments	4	23
Derivative financial instruments	38,311	11,953
Total Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	1,353,125	542,333
Available for sale securities	4,893,996	4,675,304
Held to maturity securities	1,562,577	1,611,004
Loans, net	21,388,867	21,044,079
Loans held for sale	67,587	55,864
Derivative financial instruments	166,869	54,446
Deposits	25,003,807	23,788,775
Federal funds purchased	330,330	195,450
Securities sold under agreements to repurchase	482,953	484,422
FHLB short-term borrowings	1,860,000	2,035,000
Long-term debt	231,044	226,098
Derivative financial instruments	49,409	21,089
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	1,353,125	542,333
Federal funds purchased	330,330	195,450
Securities sold under agreements to repurchase	482,953	484,422
FHLB short-term borrowings	1,860,000	2,035,000
Total Fair Value [Member] \| Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available for sale securities	4,893,996	4,675,304
Held to maturity securities	1,562,577	1,611,004
Loans, net	281,121	182,377
Loans held for sale	67,587	55,864
Derivative financial instruments	166,869	54,446
Long-term debt	231,044	226,098
Derivative financial instruments	44,064	15,385
Total Fair Value [Member] \| Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Loans, net	21,107,746	20,861,702
Deposits	25,003,807	23,786,775
Derivative financial instruments	5,345	5,704
Carrying Amount [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	1,353,125	542,333
Available for sale securities	4,893,996	4,675,304
Held to maturity securities	1,480,494	1,568,009
Loans, net	21,089,678	21,021,504
Loans held for sale	67,587	55,864
Derivative financial instruments	166,869	54,446
Deposits	25,008,496	23,803,575
Federal funds purchased	330,330	195,450
Securities sold under agreements to repurchase	482,953	484,422
FHLB short-term borrowings	1,860,000	2,035,000
Long-term debt	225,606	233,462
Derivative financial instruments	$ 49,409	$ 21,089

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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- Definition

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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