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INVESTMENT SECURITIES, Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands | Mar. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
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Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 218 | 117
12 months or longer | 3 | 3
Total | 221 | 120
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,634,895 | $ 913,612
12 months or longer | 13 | 14
Total | 1,634,908 | 913,626
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 179,439 | 16,788
12 months or longer | 0 | 0
Total | 179,439 | $ 16,788
Available-for-sale, allowance for credit losses | 0 |
Accrued interest and fees receivable | 20,900 |
Available-for-sale Securities [Member] |  |
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Accrued interest and fees receivable | $ 12,000 |
U.S. Treasury and agency securities |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 31 | 13
12 months or longer | 0 | 0
Total | 31 | 13
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,440,822 | $ 831,015
12 months or longer | 0 | 0
Total | 1,440,822 | 831,015
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 174,454 | 16,595
12 months or longer | 0 | 0
Total | $ 174,454 | $ 16,595
Obligations of state and political subdivisions |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 17 | 1
12 months or longer | 0 | 0
Total | 17 | 1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 7,028 | $ 358
12 months or longer | 0 | 0
Total | 7,028 | 358
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 9 | 0
12 months or longer | 0 | 0
Total | $ 9 | $ 0
Government agency mortgage-backed securities |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 162 | 89
12 months or longer | 2 | 2
Total | 164 | 91
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 184,325 | $ 75,992
12 months or longer | 13 | 14
Total | 184,338 | 76,006
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 4,969 | 182
12 months or longer | 0 | 0
Total | $ 4,969 | $ 182
Corporate debt securities |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months |  | 1
12 months or longer |  | 0
Total |  | 1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months |  | $ 1,001
12 months or longer |  | 0
Total |  | 1,001
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months |  | 1
12 months or longer |  | 0
Total |  | $ 1
Government agency collateralized mortgage obligations |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 8 | 13
12 months or longer | 1 | 1
Total | 9 | 14
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 2,720 | $ 5,246
12 months or longer | 0 | 0
Total | 2,720 | 5,246
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 7 | 10
12 months or longer | 0 | 0
Total | $ 7 | $ 10

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