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FAIR VALUE, Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 2,939,944 |
Equity securities | 504 | $ 0
Marketable equity securities |  | 526
Available-for-sale investment securities | 2,939,944 | 3,031,088
Derivative asset | 128 | 89
U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,013,962 | 2,054,071
Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 491,484 | 528,956
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 355,897 | 357,538
Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,563 | 2,623
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 76,038 | 87,374
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,939,944 |
Equity securities | 504 |
Marketable equity securities |  | 526
Available-for-sale investment securities |  | 3,031,088
Mortgage loans held for sale | 1,281 | 461
Total | 2,941,915 | 3,031,802
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 128 | 89
Recurring [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 64 | 4
Recurring [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 1,107 | 1,064
Derivative liability | (1,113) | (904)
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,013,962 | 2,054,071
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 491,484 | 528,956
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 355,897 | 357,538
Recurring [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,563 | 2,623
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 76,038 | 87,374
Recurring [Member] | Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,876,745 |
Equity securities | 504 |
Marketable equity securities |  | 526
Available-for-sale investment securities |  | 1,909,816
Mortgage loans held for sale | 0 | 0
Total | 1,877,249 | 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 | 1,876,745 | 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 2 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,063,199 |
Equity securities | 0 |
Marketable equity securities |  | 0
Available-for-sale investment securities |  | 1,121,272
Mortgage loans held for sale | 1,281 | 461
Total | 1,064,538 | 1,121,897
Mortgage loans held for sale, at principal value | 1,300 |
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 | 64 | 4
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 1,107 | 1,064
Derivative liability | (1,113) | (904)
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 137,217 | 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 | 491,484 | 528,956
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 355,897 | 357,538
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,563 | 2,623
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 76,038 | 87,374
Recurring [Member] | Level 3 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 |
Equity securities | 0 |
Marketable equity securities |  | 0
Available-for-sale investment securities |  | 0
Mortgage loans held for sale | 0 | 0
Total | 128 | 89
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 128 | 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

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