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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, 2020 | Dec. 31, 2019
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 3,141,882 |
Equity securities | 421 | $ 451
Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 530 |
U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,139,079 | 2,043,759
Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 501,708 | 512,208
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 448,248 | 432,862
Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,508 | 2,528
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 50,339 | 53,071
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,141,882 | 3,044,428
Equity securities | 421 | 451
Mortgage loans held for sale | 3,272 |
Total | 3,146,695 | 3,045,144
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 530 |
Recurring [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative liability | 26 |
Recurring [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 1,825 | 851
Derivative liability | (1,261) | (586)
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,139,079 | 2,043,759
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 501,708 | 512,208
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 448,248 | 432,862
Recurring [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,508 | 2,528
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 50,339 | 53,071
Recurring [Member] | Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,986,718 | 1,878,705
Equity securities | 421 | 451
Mortgage loans held for sale | 0 |
Total | 1,987,139 | 1,879,156
Recurring [Member] | Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 |
Recurring [Member] | Level 1 [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative liability | 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 | 1,986,718 | 1,878,705
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 | 1,155,164 | 1,165,723
Equity securities | 0 | 0
Mortgage loans held for sale | 3,272 |
Total | 1,159,026 | 1,165,988
Mortgage loans held for sale, at principal value | 3,000 | 0
Recurring [Member] | Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 |
Recurring [Member] | Level 2 [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative liability | 26 |
Recurring [Member] | Level 2 [Member] | Interest Rate Swap Agreements [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 1,825 | 851
Derivative liability | (1,261) | (586)
Recurring [Member] | Level 2 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 152,361 | 165,054
Recurring [Member] | Level 2 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 501,708 | 512,208
Recurring [Member] | Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 448,248 | 432,862
Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,508 | 2,528
Recurring [Member] | Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 50,339 | 53,071
Recurring [Member] | Level 3 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Equity securities | 0 | 0
Mortgage loans held for sale | 0 |
Total | 530 | 0
Recurring [Member] | Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 530 |
Recurring [Member] | Level 3 [Member] | Forward Sales Commitments [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative liability | 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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