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FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 3,031,088 | $ 2,748,656
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,031,088 | 2,748,656
Mortgage loans held for sale | 461 | 2,416
Total | 3,031,802 | 2,751,129
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 89 | 54
Recurring [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 4 | 3
Recurring [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 1,064 |
Derivative liability | (904) |
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,054,071 | 1,902,762
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 528,956 | 594,990
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 357,538 | 235,230
Recurring [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,623 | 5,687
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 87,374 | 9,535
Recurring [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 526 | 452
Recurring [Member] | Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,909,816 | 1,903,214
Mortgage loans held for sale | 0 | 0
Total | 1,909,816 | 1,903,214
Recurring [Member] | Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Level 1 [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Level 1 [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 |
Derivative liability | 0 |
Recurring [Member] | Level 1 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,909,290 | 1,902,762
Recurring [Member] | Level 1 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 1 [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 526 | 452
Recurring [Member] | Level 2 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,121,272 | 845,442
Mortgage loans held for sale | 461 | 2,416
Total | 1,121,897 | 847,861
Mortgage loans held for sale, at principal value | 500 |
Mortgage loans held for sale, gross unrealized gain (Loss) | 13 |
Recurring [Member] | Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Level 2 [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 4 | 3
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 1,064 |
Derivative liability | (904) |
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 144,781 | 0
Recurring [Member] | Level 2 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 528,956 | 594,990
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 357,538 | 235,230
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,623 | 5,687
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 87,374 | 9,535
Recurring [Member] | Level 2 [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Mortgage loans held for sale | 0 | 0
Total | 89 | 54
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 89 | 54
Recurring [Member] | Level 3 [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Level 3 [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 |
Derivative liability | 0 |
Recurring [Member] | Level 3 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Level 3 [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 0 | $ 0

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