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INVESTMENT AND MORTGAGE-BACKED SECURITIES (Tables) | 3 Months Ended
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Mar. 31, 2021
INVESTMENT AND MORTGAGE-BACKED SECURITIES |
Major Categories of Securities Owned by Entity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | March 31, 2021
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(In thousands) |  | Cost |  | Gains |  | Losses |  | Value
Securities available-for-sale: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Agency notes | ​ | $ | 72,500 | ​ | $ | — | ​ | $ | (2,930) | ​ | $ | 69,570
Treasury securities | ​ | ​ | 92,547 | ​ | ​ | — | ​ | ​ | (275) | ​ | ​ | 92,272
Corporate securities | ​ |  | 95,786 | ​ |  | 3,367 | ​ |  | (420) | ​ |  | 98,733
Pass-through MBS issued by GSEs | ​ |  | 321,442 | ​ |  | 7,053 | ​ |  | (4,788) | ​ |  | 323,707
Agency Collateralized Mortgage Obligations ("CMOs") | ​ |  | 522,841 | ​ |  | 6,305 | ​ |  | (6,324) | ​ |  | 522,822
State and municipal obligations | ​ | ​ | 45,498 | ​ | ​ | 179 | ​ | ​ | (288) | ​ | ​ | 45,389
Total securities available-for-sale | ​ | $ | 1,150,614 | ​ | $ | 16,904 | ​ | $ | (15,025) | ​ | $ | 1,152,493
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2020
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(In thousands) |  | Cost |  | Gains |  | Losses |  | Value
Securities available-for-sale: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Agency notes | ​ | $ | 47,500 | ​ | $ | 12 | ​ | $ | (91) | ​ | $ | 47,421
Corporate securities | ​ |  | 62,021 | ​ |  | 2,440 | ​ |  | — | ​ |  | 64,461
Pass-through MBS issued by GSEs | ​ |  | 135,842 | ​ |  | 7,672 | ​ |  | (31) | ​ |  | 143,483
Agency CMOs | ​ |  | 274,898 | ​ |  | 8,674 | ​ |  | (76) | ​ |  | 283,496
Total securities available-for-sale | ​ | $ | 520,261 | ​ | $ | 18,798 | ​ | $ | (198) | ​ | $ | 538,861
Schedule of amortized cost and fair value of debt securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ |
​ | ​ | March 31, 2021
​ | ​ | Amortized | ​ | Fair
(In thousands) | ​ | Cost | ​ | Value
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​
Within one year | ​ | $ | 2,123 | ​ | $ | 2,130
One to five years | ​ | ​ | 116,049 | ​ | ​ | 115,878
Five to ten years | ​ | ​ | 183,014 | ​ | ​ | 182,961
Beyond ten years | ​ | ​ | 5,145 | ​ | ​ | 4,995
Pass-through MBS issued by GSEs and agency CMO | ​ | ​ | 844,283 | ​ | ​ | 846,529
Total | ​ | $ | 1,150,614 | ​ | $ | 1,152,493
Summary of Sale of Available-for-sale Securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31,
(In thousands) | ​ | 2021 |  | 2020
Corporate Securities: | ​ | ​ | ​ | ​ | ​ | ​
Proceeds | ​ | $ | 46,754 | ​ | $ | —
Gross gains | ​ | ​ | 709 | ​ | ​ | —
Tax expense on gain | ​ | ​ | 225 | ​ | ​ | —
Gross losses | ​ | ​ | 41 | ​ | ​ | —
Tax benefit on loss | ​ | ​ | 13 | ​ | ​ | —
Pass through MBS issued by GSEs: | ​ | ​ |  |  | ​ |
Proceeds | ​ | ​ | 26,823 | ​ | ​ | —
Gross gains | ​ |  | 187 | ​ |  | —
Tax expense on gain | ​ |  | 59 | ​ |  | —
Gross losses | ​ |  | 35 | ​ |  | —
Tax benefit on loss | ​ |  | 11 | ​ |  | —
Agency CMOs: | ​ |  |  | ​ |  |
Proceeds | ​ |  | 41,324 | ​ |  | 4,199
Gross gains | ​ |  | 268 | ​ |  | 8
Tax expense on gain | ​ |  | 85 | ​ |  | 3
Gross losses | ​ | ​ | 44 | ​ | ​ | —
Tax benefit on loss | ​ | ​ | 14 | ​ | ​ | —
State and municipal obligations: | ​ |  |  | ​ |  |
Proceeds | ​ |  | 19,657 | ​ |  | —
Gross gains | ​ |  | 143 | ​ |  | —
Tax expense on gain | ​ |  | 45 | ​ |  | —
Sales of Marketable Equity Securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31,
(In thousands) | ​ | 2021 |  | 2020
Proceeds: | ​ | ​ |  |  | ​ |
Marketable equity securities | ​ | $ | 6,101 | ​ | $ | 137
Gross Unrealized Losses and Fair Value of Investment Securities by Investment Category and Length of Time in a Continuous Unrealized Loss Position | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | March 31, 2021
​ | ​ | Less than 12 | ​ | 12 Consecutive | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Consecutive Months | ​ | Months or Longer | ​ | Total
​ | ​ | Fair | ​ | Unrealized | ​ | Fair | ​ | Unrealized | ​ | Fair | ​ | Unrealized
(In thousands) |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Securities available-for-sale: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Agency notes | ​ | $ | 69,570 | ​ | $ | 2,930 | ​ | $ | — | ​ | $ | — | ​ | $ | 69,570 | ​ | $ | 2,930
Treasury securities | ​ | ​ | 92,272 | ​ | ​ | 275 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 92,272 | ​ | ​ | 275
Corporate securities | ​ | ​ | 18,278 | ​ | ​ | 420 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 18,278 | ​ | ​ | 420
Pass-through MBS issued by GSEs | ​ | ​ | 181,917 | ​ | ​ | 4,788 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 181,917 | ​ | ​ | 4,788
Agency CMOs | ​ | ​ | 239,145 | ​ | ​ | 6,324 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 239,145 | ​ | ​ | 6,324
State and municipal obligations | ​ |  | 19,004 | ​ |  | 288 | ​ |  | — | ​ |  | — | ​ |  | 19,004 | ​ |  | 288
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2020
​ | ​ | Less than 12 | ​ | 12 Consecutive | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Consecutive Months | ​ | Months or Longer | ​ | Total
​ | ​ | Fair | ​ | Unrealized | ​ | Fair | ​ | Unrealized | ​ | Fair | ​ | Unrealized
(In thousands) |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Securities available-for-sale: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Agency notes | ​ | $ | 22,409 | ​ | $ | 91 | ​ | $ | — | ​ | $ | — | ​ | $ | 22,409 | ​ | $ | 91
Corporate securities | ​ |  | — | ​ |  | — | ​ |  | — | ​ |  | — | ​ |  | — | ​ |  | —
Pass-through MBS issued by GSEs | ​ |  | 5,007 | ​ |  | 31 | ​ |  | — | ​ |  | — | ​ |  | 5,007 | ​ |  | 31
Agency CMOs | ​ |  | 6,563 | ​ |  | 30 | ​ |  | 4,954 | ​ |  | 46 | ​ |  | 11,517 | ​ |  | 76

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