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INVESTMENT AND MORTGAGE-BACKED SECURITIES - Narrative (Details) - USD ($) | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020 | Feb. 01, 2021
INVESTMENT AND MORTGAGE-BACKED SECURITIES |  |  |  |  |  |
Acquired securities available-for-sale on the Merger Date |  |  |  |  |  | $ 652,000,000.0
Investment securities held-to-maturity, fair value | $ 40,303,000 |  | $ 40,303,000 |  | $ 0 |
Securities pledged as collateral, carrying amount | $ 684,000,000.0 |  | $ 684,000,000.0 |  | $ 99,400,000 |
Number of holdings of securities of any one issuer in an amount greater than 10% of stockholders equity | 0 |  | 0 |  | 0 |
Threshold for disclosure percentage | 10.00% |  |  |  | 10.00% |
Net gain on equity securities | $ 0 | $ 175,000 | $ 131,000 | $ 139,000 |  |

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