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INVESTMENT AND MORTGAGE-BACKED SECURITIES (Tables) | 12 Months Ended
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Dec. 31, 2021
INVESTMENT AND MORTGAGE-BACKED SECURITIES |
Major Categories of Securities Owned by Entity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2021
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(In thousands) |  | Cost |  | Gains |  | Losses |  | Value
Securities available-for-sale: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Agency notes | ​ | $ | 82,476 | ​ | $ | — | ​ | $ | (2,222) | ​ | $ | 80,254
Treasury securities | ​ | ​ | 247,916 | ​ | ​ | — | ​ | ​ | (3,147) | ​ | ​ | 244,769
Corporate securities | ​ |  | 148,430 | ​ |  | 4,354 | ​ |  | (754) | ​ |  | 152,030
Pass-through mortgage-backed securities ("MBS") issued by government sponsored entities ("GSEs") | ​ |  | 528,749 | ​ |  | 4,271 | ​ |  | (6,566) | ​ |  | 526,454
Agency collateralized mortgage obligations ("CMOs") | ​ |  | 527,348 | ​ |  | 2,705 | ​ |  | (8,795) | ​ |  | 521,258
State and municipal obligations | ​ | ​ | 39,175 | ​ | ​ | 73 | ​ | ​ | (302) | ​ | ​ | 38,946
Total securities available-for-sale | ​ | $ | 1,574,094 | ​ | $ | 11,403 | ​ | $ | (21,786) | ​ | $ | 1,563,711
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 31, 2021
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrecognized | ​ | Unrecognized | ​ | Fair
(In thousands) |  | Cost |  | Gains |  | Losses |  | Value
Securities held-to-maturity: |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Pass-through MBS issued by GSEs | ​ | $ | 118,382 | ​ | $ | 59 | ​ | $ | (1,141) | ​ | $ | 117,300
Agency CMOs | ​ |  | 60,927 | ​ |  | — | ​ |  | (873) | ​ |  | 60,054
Total securities held-to-maturity | ​ | $ | 179,309 | ​ | $ | 59 | ​ | $ | (2,014) | ​ | $ | 177,354
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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 | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ |
​ | ​ | December 31, 2021
​ | ​ | Amortized | ​ | Fair
(In thousands) | ​ | Cost | ​ | Value
Available-for-sale | ​ | ​ | ​ | ​ | ​ | ​
Within one year | ​ | $ | 852 | ​ | $ | 858
One to five years | ​ | ​ | 281,148 | ​ | ​ | 277,877
Five to ten years | ​ | ​ | 222,851 | ​ | ​ | 224,137
Beyond ten years | ​ | ​ | 13,145 | ​ | ​ | 13,127
Pass-through MBS issued by GSEs and agency CMO | ​ | ​ | 1,056,098 | ​ | ​ | 1,047,712
Total | ​ | $ | 1,574,094 | ​ | $ | 1,563,711
​ | ​ | ​ | ​ | ​ | ​ | ​
Held-to-maturity | ​ | ​ | ​ | ​ | ​ | ​
Pass-through MBS issued by GSEs and agency CMO | ​ | $ | 179,309 | ​ | $ | 177,354
Total | ​ | $ | 179,309 | ​ | $ | 177,354
Summary of Sale of Available-for-sale Securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
(In thousands) |  | 2021 |  | 2020 |  | 2019
Securities available-for-sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Proceeds | ​ | $ | 138,077 | ​ | $ | 94,252 | ​ | $ | 148,857
Gross gains | ​ | ​ | 1,327 | ​ | ​ | 4,592 | ​ | ​ | 551
Tax expense on gains | ​ | ​ | 421 | ​ | ​ | 1,444 | ​ | ​ | 175
Gross losses | ​ | ​ | 120 | ​ | ​ | — | ​ | ​ | 520
Tax benefit on losses | ​ | ​ | 38 | ​ | ​ | — | ​ | ​ | 166
​
Sales of Marketable Equity Securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
(In thousands) | ​ | 2021 |  | 2020 |  | 2019
Proceeds: | ​ | ​ |  |  | ​ |  |  | ​ |
Marketable equity securities | ​ | $ | 6,101 | ​ | $ | 546 | ​ | $ | 570
Gross Unrealized Losses and Fair Value of Investment Securities by Investment Category and Length of Time in a Continuous Unrealized Loss Position | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | December 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 | ​ | $ | 58,607 | ​ | $ | 1,369 | ​ | $ | 21,647 | ​ | $ | 853 | ​ | $ | 80,254 | ​ | $ | 2,222
Treasury securities | ​ | ​ | 244,769 | ​ | ​ | 3,147 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 244,769 | ​ | ​ | 3,147
Corporate securities | ​ | ​ | 37,620 | ​ | ​ | 754 | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 37,620 | ​ | ​ | 754
Pass-through MBS issued by GSEs | ​ | ​ | 422,634 | ​ | ​ | 6,333 | ​ | ​ | 4,748 | ​ | ​ | 233 | ​ | ​ | 427,382 | ​ | ​ | 6,566
Agency CMOs | ​ | ​ | 349,879 | ​ | ​ | 8,672 | ​ | ​ | 3,182 | ​ | ​ | 123 | ​ | ​ | 353,061 | ​ | ​ | 8,795
State and municipal obligations | ​ |  | 18,887 | ​ |  | 302 | ​ |  | — | ​ |  | — | ​ | ​ | 18,887 | ​ | ​ | 302
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Securities held-to-maturity |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |
Pass-through MBS issued by GSEs | ​ | $ | 97,328 | ​ | $ | 1,141 | ​ | $ | — | ​ | $ | — | ​ | $ | 97,328 | ​ | $ | 1,141
Agency CMOs | ​ | ​ | 60,054 | ​ | ​ | 873 | ​ |  | — | ​ |  | — | ​ | ​ | 60,054 | ​ |  | 873
​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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
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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