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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, 2021 | Dec. 31, 2020
--- | --- | ---
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 4,012,739 |
Equity securities | 469 | $ 445
U.S. Treasury and agency securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,025,072 | 2,501,382
Obligations of state and political subdivisions |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 426,152 | 475,660
Government agency mortgage-backed securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 529,139 | 522,638
Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,094 | 4,635
Government agency collateralized mortgage obligations |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 29,282 | 43,577
Significant Unobservable Inputs, Level 3 [Member] | Commitments to originate real estate loans for sale |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 130 | 14
Recurring [Member] |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 4,012,739 | 3,547,892
Equity securities | 469 | 445
Mortgage loans held for sale | 1,340 | 1,622
Total | 4,015,074 | 3,550,473
Recurring [Member] | Commitments to originate real estate loans for sale |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 130 | 14
Recurring [Member] | Forward sales commitments |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 31 | 2
Recurring [Member] | Interest rate swap agreements |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 372 | 1,572
Derivative liability | (7) | (1,074)
Recurring [Member] | U.S. Treasury and agency securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,025,072 | 2,501,382
Recurring [Member] | Obligations of state and political subdivisions |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 426,152 | 475,660
Recurring [Member] | Government agency mortgage-backed securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 529,139 | 522,638
Recurring [Member] | Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,094 | 4,635
Recurring [Member] | Government agency collateralized mortgage obligations |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 29,282 | 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 | 2,909,475 | 2,359,912
Equity securities | 469 | 445
Mortgage loans held for sale | 0 | 0
Total | 2,909,944 | 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 |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 2,909,475 | 2,359,912
Recurring [Member] | Quoted Prices in Active Markets For Identical Assets, Level 1 [Member] | Obligations of state and political subdivisions |  |
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 |  |
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 |  |
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 | 1,103,264 | 1,187,980
Equity securities | 0 | 0
Mortgage loans held for sale | 1,340 | 1,622
Total | 1,105,000 | 1,190,102
Mortgage loans held for sale | 1,300 | 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 | 31 | 2
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Interest rate swap agreements |  |
Available-for-sale investment securities [Abstract] |  |
Derivative asset | 372 | 1,572
Derivative liability | (7) | (1,074)
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | U.S. Treasury and agency securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 115,597 | 141,470
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Obligations of state and political subdivisions |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 426,152 | 475,660
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government agency mortgage-backed securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 529,139 | 522,638
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Corporate debt securities |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 3,094 | 4,635
Recurring [Member] | Significant Observable Inputs, Level 2 [Member] | Government agency collateralized mortgage obligations |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | 29,282 | 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 | 130 | 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 | 130 | 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 |  |
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 |  |
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 |  |
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 |  |
Available-for-sale investment securities [Abstract] |  |
Available-for-sale investment securities | $ 0 | $ 0

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