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INVESTMENT SECURITIES - Investment Securities in a Continuous Unrealized Loss Position (Details) $ in Thousands | Jun. 30, 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 | 162 | 117
12 months or longer | 9 | 3
Total | 171 | 120
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,446,539 | $ 913,612
12 months or longer | 1,328 | 14
Total | 1,447,867 | 913,626
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 112,993 | 16,788
12 months or longer | 3 | 0
Total | 112,996 | $ 16,788
Available-for-sale, allowance for credit losses | 0 |
Accrued interest and fees receivable | 19,000 |
Available-for-sale Securities [Member] |  |
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Accrued interest and fees receivable | $ 14,500 |
U.S. Treasury and agency securities |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 29 | 13
12 months or longer | 0 | 0
Total | 29 | 13
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,272,423 | $ 831,015
12 months or longer | 0 | 0
Total | 1,272,423 | 831,015
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 110,075 | 16,595
12 months or longer | 0 | 0
Total | $ 110,075 | $ 16,595
Obligations of state and political subdivisions |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 4 | 1
12 months or longer | 0 | 0
Total | 4 | 1
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,651 | $ 358
12 months or longer | 0 | 0
Total | 1,651 | 358
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 1 | 0
12 months or longer | 0 | 0
Total | $ 1 | $ 0
Government agency mortgage-backed securities |  |
Available-for-Sale Securities in Unrealized Loss Positions, Number of Positions [Abstract] |  |
Less than 12 months | 126 | 89
12 months or longer | 6 | 2
Total | 132 | 91
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 170,661 | $ 75,992
12 months or longer | 1,146 | 14
Total | 171,807 | 76,006
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 2,914 | 182
12 months or longer | 2 | 0
Total | $ 2,916 | $ 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 | 3 | 13
12 months or longer | 3 | 1
Total | 6 | 14
Available-for-Sale Securities, in Unrealized Loss Position, Fair Value [Abstract] |  |
Less than 12 months | $ 1,804 | $ 5,246
12 months or longer | 182 | 0
Total | 1,986 | 5,246
Available-for-Sale Securities, in Unrealized Loss Position, Gross Unrealized Losses [Abstract] |  |
Less than 12 months | 3 | 10
12 months or longer | 1 | 0
Total | $ 4 | $ 10

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