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FAIR VALUE - Financial Assets and Liabilities Accounted for at Fair Value On a Recurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 4,358,371 |
Equity securities | 459 | $ 445
Derivative asset |  | 14
U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,393,260 | 2,501,382
Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 421,965 | 475,660
Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 510,676 | 522,638
Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 8,078 | 4,635
Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 24,392 | 43,577
Significant Unobservable Inputs, Level 3 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 45 |
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 4,358,371 | 3,547,892
Equity securities | 459 | 445
Mortgage loans held for sale | 117 | 1,622
Total | 4,359,375 | 3,550,473
Recurring [Member] | Commitments to Originate Real Estate Loans for Sale |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 45 | 14
Recurring [Member] | Forward Sales Commitments |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 53 | 2
Recurring [Member] | Interest Rate Swap Agreements |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 335 | 1,572
Derivative liability | (5) | 1,074
Recurring [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,393,260 | 2,501,382
Recurring [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 421,965 | 475,660
Recurring [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 510,676 | 522,638
Recurring [Member] | Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 8,078 | 4,635
Recurring [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 24,392 | 43,577
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,293,134 | 2,359,912
Equity securities | 459 | 445
Mortgage loans held for sale | 0 | 0
Total | 3,293,593 | 2,360,357
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Commitments to Originate Real Estate Loans for Sale |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Forward Sales Commitments |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Interest Rate Swap Agreements |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Derivative liability | 0 | 0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,293,134 | 2,359,912
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, 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] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 1,065,237 | 1,187,980
Equity securities | 0 | 0
Mortgage loans held for sale | 117 | 1,622
Total | 1,065,737 | 1,190,102
Mortgage loans held for sale | 100 | 1,600
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Commitments to Originate Real Estate Loans for Sale |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Forward Sales Commitments |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 53 | 2
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Interest Rate Swap Agreements |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 335 | 1,572
Derivative liability | (5) | 1,074
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | U.S. Treasury and Agency Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 100,126 | 141,470
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Obligations of State and Political Subdivisions [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 421,965 | 475,660
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 510,676 | 522,638
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 8,078 | 4,635
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government Agency Collateralized Mortgage Obligations [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 24,392 | 43,577
Recurring [Member] | Significant Unobservable Inputs, 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 | 0
Total | 45 | 14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Commitments to Originate Real Estate Loans for Sale |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 45 | 14
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Forward Sales Commitments |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Interest Rate Swap Agreements |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 0 | 0
Derivative liability | 0 | 0
Recurring [Member] | Significant Unobservable Inputs, 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] | Significant Unobservable Inputs, 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] | Significant Unobservable Inputs, Level 3 [Member] | Government Agency Mortgage-Backed Securities [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Significant Unobservable Inputs, Level 3 [Member] | Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 0 | 0
Recurring [Member] | Significant Unobservable Inputs, 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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