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INVESTMENT AND MORTGAGE-BACKED SECURITIES - Major Categories of Securities Owned (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020 | Feb. 01, 2021
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | $ 1,250,962,000 |  | $ 1,250,962,000 |  | $ 520,261,000 |
Gross Unrealized Gains | 18,165,000 |  | 18,165,000 |  | 18,798,000 |
Gross Unrealized Losses | (7,339,000) |  | (7,339,000) |  | (198,000) |
Fair Value | 1,261,788,000 |  | 1,261,788,000 |  | 538,861,000 |
Available for sale securities | 863,239,000 |  | 863,239,000 |  | 426,979,000 |
Acquired securities available-for-sale on the Merger Date |  |  |  |  |  | $ 652,000,000.0
Securities pledged as collateral | $ 505,500,000 |  | $ 505,500,000 |  | $ 99,400,000 |
Number of holdings of securities of any one issuer, other than the U.S. Government and its agencies, in an amount greater than 10% of stockholders equity | 0 |  | 0 |  | 0 |
Threshold for disclosure percentage | 10.00% |  |  |  | 10.00% |
Sales of Available-for-sale Securities: |  |  |  |  |  |
Proceeds |  |  | $ 138,077,000 | $ 62,797,000 |  |
Proceeds: |  |  |  |  |  |
Marketable equity securities |  |  | 0 | 273,000 |  |
Net gain (loss) on equity securities |  | $ 436,000 | 131,000 | (36,000) |  |
Marketable Equity Securities [Member] |  |  |  |  |  |
Proceeds: |  |  |  |  |  |
Marketable equity securities | $ 0 | 136,000 | 6,101,000 | 273,000 |  |
Net gain (loss) on equity securities | 0 | 436,000 | 131,000 | (36,000) |  |
Agency Notes |  |  |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | 82,474,000 |  | 82,474,000 |  | $ 47,500,000 |
Gross Unrealized Gains |  |  |  |  | 12,000 |
Gross Unrealized Losses | (1,439,000) |  | (1,439,000) |  | (91,000) |
Fair Value | 81,035,000 |  | 81,035,000 |  | 47,421,000 |
Sales of Available-for-sale Securities: |  |  |  |  |  |
Proceeds |  | 273,000 |  | 273,000 |  |
Treasury Securities |  |  |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | 167,984,000 |  | 167,984,000 |  |  |
Gross Unrealized Gains | 114,000 |  | 114,000 |  |  |
Gross Unrealized Losses | (100,000) |  | (100,000) |  |  |
Fair Value | 167,998,000 |  | 167,998,000 |  |  |
Corporate Securities |  |  |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | 103,740,000 |  | 103,740,000 |  | 62,021,000 |
Gross Unrealized Gains | 4,380,000 |  | 4,380,000 |  | 2,440,000 |
Gross Unrealized Losses | (56,000) |  | (56,000) |  |  |
Fair Value | 108,064,000 |  | 108,064,000 |  | 64,461,000 |
Sales of Available-for-sale Securities: |  |  |  |  |  |
Proceeds | 3,519,000 | 25,403,000 | 50,273,000 | 25,403,000 |  |
Gross gains | 20,000 | 1,344,000 | 729,000 | 1,344,000 |  |
Tax expense on gain | 6,000 | 423,000 | 232,000 | 423,000 |  |
Gross losses |  |  | 41,000 |  |  |
Tax benefit on loss |  |  | 13,000 |  |  |
Pass-through MBS issued by GSEs |  |  |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | 326,786,000 |  | 326,786,000 |  | 135,842,000 |
Gross Unrealized Gains | 6,908,000 |  | 6,908,000 |  | 7,672,000 |
Gross Unrealized Losses | (1,945,000) |  | (1,945,000) |  | (31,000) |
Fair Value | 331,749,000 |  | 331,749,000 |  | 143,483,000 |
Sales of Available-for-sale Securities: |  |  |  |  |  |
Proceeds |  | 33,195,000 | 26,823,000 | 33,195,000 |  |
Gross gains |  | 1,790,000 | 187,000 | 1,790,000 |  |
Tax expense on gain |  | 563,000 | 59,000 | 563,000 |  |
Gross losses | 0 | 0 | 35,000 | 0 |  |
Tax benefit on loss | 0 | 0 | 11,000 | 0 |  |
Agency CMOs [Member] |  |  |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | 528,649,000 |  | 528,649,000 |  | 274,898,000 |
Gross Unrealized Gains | 6,532,000 |  | 6,532,000 |  | 8,674,000 |
Gross Unrealized Losses | (3,691,000) |  | (3,691,000) |  | (76,000) |
Fair Value | 531,490,000 |  | 531,490,000 |  | $ 283,496,000 |
Sales of Available-for-sale Securities: |  |  |  |  |  |
Proceeds |  |  | 41,324,000 | 4,199,000 |  |
Gross gains |  |  | 268,000 | 8,000 |  |
Tax expense on gain | 0 | 0 | 85,000 | 3,000 |  |
Gross losses | 0 | 0 | 44,000 | 0 |  |
Tax benefit on loss | 0 | 0 | 14,000 | 0 |  |
State and municipal obligations |  |  |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |  |  |
Amortized Cost | 41,329,000 |  | 41,329,000 |  |  |
Gross Unrealized Gains | 231,000 |  | 231,000 |  |  |
Gross Unrealized Losses | (108,000) |  | (108,000) |  |  |
Fair Value | 41,452,000 |  | 41,452,000 |  |  |
Sales of Available-for-sale Securities: |  |  |  |  |  |
Proceeds | 0 | 0 | 19,657,000 | 0 |  |
Gross gains | 0 | 0 | 143,000 | 0 |  |
Tax expense on gain | $ 0 | $ 0 | $ 45,000 | $ 0 |  |

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