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FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 2,987,290 | $ 3,031,088
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,987,290 | 3,031,088
Mortgage loans held for sale | 628 | 461
Total | 2,988,027 | 3,031,802
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 55 | 89
Recurring [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 10 | 4
Recurring [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 937 | 1,064
Derivative liability | (893) | (904)
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,027,914 | 2,054,071
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 510,308 | 528,956
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 364,539 | 357,538
Recurring [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,579 | 2,623
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 81,429 | 87,374
Recurring [Member] | Marketable Equity Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 521 | 526
Recurring [Member] | Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,885,536 | 1,909,816
Mortgage loans held for sale | 0 | 0
Total | 1,885,536 | 1,909,816
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 | 0
Derivative liability | 0 | 0
Recurring [Member] | Level 1 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,885,015 | 1,909,290
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 | 521 | 526
Recurring [Member] | Level 2 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,101,754 | 1,121,272
Mortgage loans held for sale | 628 | 461
Total | 1,102,436 | 1,121,897
Mortgage loans held for sale, at principal value | 600 |
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 | 10 | 4
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 937 | 1,064
Derivative liability | (893) | (904)
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 142,899 | 144,781
Recurring [Member] | Level 2 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 510,308 | 528,956
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 364,539 | 357,538
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,579 | 2,623
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 81,429 | 87,374
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 | 55 | 89
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 55 | 89
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 | 0
Derivative liability | 0 | 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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