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INVESTMENT AND MORTGAGE-BACKED SECURITIES - Major Categories of Securities Owned (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020 | Feb. 01, 2021
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | $ 1,150,614 |  | $ 520,261 |
Gross Unrealized Gains | 16,904 |  | 18,798 |
Gross Unrealized Losses | (15,025) |  | (198) |
Fair Value | 1,152,493 |  | 538,861 |
Available for sale securities | 846,529 |  | 426,979 |
Acquired securities available-for-sale on the Merger Date |  |  |  | $ 652,000
Securities pledged as collateral | $ 637,100 |  | $ 99,400 |
Threshold for disclosure percentage | 10.00% |  | 10.00% |
Sales of Available-for-sale Securities: |  |  |  |
Proceeds | $ 134,558 | $ 4,199 |  |
Proceeds: |  |  |  |
Marketable equity securities | 6,101 | 137 |  |
Net gain (loss) on equity securities | 131 | (472) |  |
Marketable Equity Securities [Member] |  |  |  |
Proceeds: |  |  |  |
Marketable equity securities | 6,101 | 137 |  |
Net gain (loss) on equity securities | 131 | 472 |  |
Agency Notes |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | 72,500 |  | $ 47,500 |
Gross Unrealized Gains |  |  | 12 |
Gross Unrealized Losses | (2,930) |  | (91) |
Fair Value | 69,570 |  | 47,421 |
Treasury Securities |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | 92,547 |  |  |
Gross Unrealized Losses | (275) |  |  |
Fair Value | 92,272 |  |  |
Corporate Securities |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | 95,786 |  | 62,021 |
Gross Unrealized Gains | 3,367 |  | 2,440 |
Gross Unrealized Losses | (420) |  |  |
Fair Value | 98,733 |  | 64,461 |
Sales of Available-for-sale Securities: |  |  |  |
Proceeds | 46,754 | 0 |  |
Gross gains | 709 | 0 |  |
Tax expense on gain | 225 | 0 |  |
Gross losses | 41 | 0 |  |
Tax benefit on loss | 13 | 0 |  |
Pass-through MBS issued by GSEs |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | 321,442 |  | 135,842 |
Gross Unrealized Gains | 7,053 |  | 7,672 |
Gross Unrealized Losses | (4,788) |  | (31) |
Fair Value | 323,707 |  | 143,483 |
Sales of Available-for-sale Securities: |  |  |  |
Proceeds | 26,823 |  |  |
Gross gains | 187 |  |  |
Tax expense on gain | 59 |  |  |
Gross losses | 35 | 0 |  |
Tax benefit on loss | 11 | 0 |  |
Agency Collateralized Mortgage Obligations ("CMOs") |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | 522,841 |  | 274,898 |
Gross Unrealized Gains | 6,305 |  | 8,674 |
Gross Unrealized Losses | (6,324) |  | (76) |
Fair Value | 522,822 |  | $ 283,496 |
Sales of Available-for-sale Securities: |  |  |  |
Proceeds | 41,324 | 4,199 |  |
Gross gains | 268 | 8 |  |
Tax expense on gain | 85 | 3 |  |
Gross losses | 44 | 0 |  |
Tax benefit on loss | 14 | 0 |  |
State and municipal obligations |  |  |  |
Securities, available-for-sale [Abstract] |  |  |  |
Amortized Cost | 45,498 |  |  |
Gross Unrealized Gains | 179 |  |  |
Gross Unrealized Losses | (288) |  |  |
Fair Value | 45,389 |  |  |
Sales of Available-for-sale Securities: |  |  |  |
Proceeds | 19,657 | 0 |  |
Gross gains | 143 | 0 |  |
Tax expense on gain | $ 45 | $ 0 |  |

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