Fair Value Measurements (Estimated Fair Values of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities	$ 2,981,673	$ 2,910,869
Held to maturity securities	3,132,200	2,977,511
Derivative financial instruments	5,760	2,148
Derivative financial instruments	1,118	3,938
Total Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	459,282	479,332
Available for sale debt securities	2,981,673	2,910,869
Held to maturity securities	3,054,469	2,962,010
Loans, net	18,839,730	18,587,508
Loans held for sale	36,047	39,865
Derivative financial instruments	11,428	14,157
Deposits	22,187,823	22,238,847
Federal funds purchased	525	140,754
Securities sold under agreements to repurchase	452,994	430,569
FHLB Short-term borrowings	1,860,671	1,132,567
Long-term debt	263,134	303,631
Derivative financial instruments	25,953	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	459,282	479,332
Federal funds purchased	525	140,754
Securities sold under agreements to repurchase	452,994	430,569
FHLB Short-term borrowings	1,860,671	1,132,567
Total Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities	2,981,673	2,910,869
Held to maturity securities	3,054,469	2,962,010
Loans, net	206,110	184,205
Loans held for sale	36,047	39,865
Derivative financial instruments	11,428	14,157
Long-term debt	263,134	303,631
Derivative financial instruments	25,953	14,389
Total Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	18,633,620	18,403,303
Deposits	22,187,823	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	459,282	479,332
Available for sale debt securities	2,981,673	2,910,869
Held to maturity securities	3,132,200	2,977,511
Loans, net	19,156,387	18,786,855
Loans held for sale	36,047	39,865
Derivative financial instruments	11,428	14,157
Deposits	22,235,338	22,253,202
Federal funds purchased	525	140,754
Securities sold under agreements to repurchase	452,994	430,569
FHLB Short-term borrowings	1,860,671	1,132,567
Long-term debt	266,009	305,513
Derivative financial instruments	$ 25,953	$ 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 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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