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INVESTMENT SECURITIES (Tables) | 6 Months Ended
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Jun. 30, 2025
INVESTMENT SECURITIES |
Schedule of amortized cost and estimated fair value of investment securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | June 30, 2025 | ​ | December 31, 2024
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
(000’s omitted) |  | Cost |  | Gains |  | Losses |  | Value |  | Cost |  | Gains |  | Losses |  | Value
Available-for-Sale Portfolio: | ​ | ​ | ​ |  | ​ |  |  | ​ |  | ​ | ​ | ​ | ​ | ​ | ​ |  | ​ |  |  | ​ |  | ​ | ​ | ​
U.S. Treasury and agency securities | ​ | $ | 2,394,285 | ​ | $ | 0 | ​ | $ | 239,341 | ​ | $ | 2,154,944 | ​ | $ | 2,389,208 | ​ | $ | 0 | ​ | $ | 305,422 | ​ | $ | 2,083,786
Obligations of state and political subdivisions | ​ |  | 421,049 | ​ |  | 272 | ​ |  | 46,081 | ​ |  | 375,240 | ​ |  | 428,204 | ​ |  | 288 | ​ |  | 41,997 | ​ |  | 386,495
Government agency mortgage-backed securities | ​ |  | 343,413 | ​ |  | 66 | ​ |  | 51,552 | ​ |  | 291,927 | ​ |  | 360,102 | ​ |  | 37 | ​ |  | 58,915 | ​ |  | 301,224
Corporate debt securities | ​ |  | 5,000 | ​ |  | 0 | ​ |  | 171 | ​ |  | 4,829 | ​ |  | 8,000 | ​ |  | 0 | ​ |  | 303 | ​ |  | 7,697
Government agency collateralized mortgage obligations | ​ |  | 5,685 | ​ |  | 0 | ​ |  | 255 | ​ |  | 5,430 | ​ |  | 6,878 | ​ |  | 0 | ​ |  | 366 | ​ |  | 6,512
Total available-for-sale portfolio | ​ | $ | 3,169,432 | ​ | $ | 338 | ​ | $ | 337,400 | ​ | $ | 2,832,370 | ​ | $ | 3,192,392 | ​ | $ | 325 | ​ | $ | 407,003 | ​ | $ | 2,785,714
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Held-to-Maturity Portfolio: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury and agency securities | ​ | $ | 1,153,480 | ​ | $ | 0 | ​ | $ | 95,300 | ​ | $ | 1,058,180 | ​ | $ | 1,138,743 | ​ | $ | 0 | ​ | $ | 123,194 | ​ | $ | 1,015,549
Government agency mortgage-backed securities | ​ | ​ | 277,511 | ​ |  | 1,251 | ​ |  | 867 | ​ |  | 277,895 | ​ | ​ | 206,412 | ​ | ​ | 448 | ​ | ​ | 2,241 | ​ | ​ | 204,619
Total held-to-maturity portfolio | ​ | $ | 1,430,991 | ​ | $ | 1,251 | ​ | $ | 96,167 | ​ | $ | 1,336,075 | ​ | $ | 1,345,155 | ​ | $ | 448 | ​ | $ | 125,435 | ​ | $ | 1,220,168
Schedule of gains and losses on equity and other securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended | ​ | Six Months Ended
​ | ​ | June 30, | ​ | June 30,
(000’s omitted) |  | 2025 |  | 2024 |  | 2025 |  | 2024
Net (loss) gain recognized on equity securities | ​ | $ | (1) | ​ | $ | 867 | ​ | $ | 244 | ​ | $ | 883
Less: Net (loss) gain recognized on equity securities sold during the period | ​ |  | 0 | ​ |  | 0 | ​ |  | 0 | ​ |  | 0
Unrealized (loss) gain recognized on equity securities still held | ​ | $ | (1) | ​ | $ | 867 | ​ | $ | 244 | ​ | $ | 883
Schedule of investment securities that have been in a continuous unrealized loss position | As of June 30, 2025 ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 Months |  | 12 Months or Longer |  | Total
​ | ​ | ​ | ​ | ​ | Gross | ​ | ​ | ​ | ​ | Gross | ​ | ​ | ​ | ​ | Gross
​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | Unrealized
(000’s omitted) |  | Fair Value |  | Losses |  | Fair Value |  | Losses |  | Fair Value |  | Losses
Available-for-Sale Portfolio: | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ | ​ | ​ | ​ |
U.S. Treasury and agency securities | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 2,154,944 | ​ | $ | 239,341 | ​ | $ | 2,154,944 | ​ | $ | 239,341
Obligations of state and political subdivisions | ​ |  | 45,983 | ​ |  | 2,364 | ​ |  | 292,583 | ​ |  | 43,717 | ​ |  | 338,566 | ​ |  | 46,081
Government agency mortgage-backed securities | ​ |  | 6,568 | ​ |  | 116 | ​ |  | 282,078 | ​ |  | 51,436 | ​ |  | 288,646 | ​ |  | 51,552
Corporate debt securities | ​ | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 4,829 | ​ | ​ | 171 | ​ | ​ | 4,829 | ​ | ​ | 171
Government agency collateralized mortgage obligations | ​ |  | 0 | ​ |  | 0 | ​ |  | 5,423 | ​ |  | 255 | ​ |  | 5,423 | ​ |  | 255
Total available-for-sale investment portfolio | ​ | $ | 52,551 | ​ | $ | 2,480 | ​ | $ | 2,739,857 | ​ | $ | 334,920 | ​ | $ | 2,792,408 | ​ | $ | 337,400
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Held-to-Maturity Portfolio: | ​ |  | ​ | ​ |  | ​ |  |  |  | ​ |  |  |  |  |  | ​ |  | ​
U.S Treasury and agency securities | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 1,058,180 | ​ | $ | 95,300 | ​ | $ | 1,058,180 | ​ | $ | 95,300
Government agency mortgage-backed securities | ​ | ​ | 113,516 | ​ | ​ | 781 |  | ​ | 17,198 | ​ | ​ | 86 |  | ​ | 130,714 | ​ | ​ | 867
Total held-to-maturity portfolio | ​ | $ | 113,516 | ​ | $ | 781 |  | $ | 1,075,378 | ​ | $ | 95,386 |  | $ | 1,188,894 | ​ | $ | 96,167
​ As of December 31, 2024 ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 Months |  | 12 Months or Longer |  | Total
​ | ​ | ​ | ​ | ​ | Gross | ​ | ​ | ​ | ​ | Gross | ​ | ​ | ​ | ​ | Gross
​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | Unrealized
(000’s omitted) |  | Fair Value |  | Losses |  | Fair Value |  | Losses |  | Fair Value |  | Losses
Available-for-Sale Portfolio: | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |
U.S. Treasury and agency securities | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 2,083,786 | ​ | $ | 305,422 | ​ | $ | 2,083,786 | ​ | $ | 305,422
Obligations of state and political subdivisions | ​ |  | 47,144 | ​ |  | 847 | ​ |  | 301,202 | ​ |  | 41,150 | ​ |  | 348,346 | ​ |  | 41,997
Government agency mortgage-backed securities | ​ |  | 7,943 | ​ |  | 221 | ​ |  | 290,786 | ​ |  | 58,694 | ​ |  | 298,729 | ​ |  | 58,915
Corporate debt securities | ​ | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 7,697 | ​ | ​ | 303 | ​ | ​ | 7,697 | ​ | ​ | 303
Government agency collateralized mortgage obligations | ​ |  | 0 | ​ |  | 0 | ​ |  | 6,501 | ​ |  | 366 | ​ |  | 6,501 | ​ |  | 366
Total available-for-sale investment portfolio | ​ | $ | 55,087 | ​ | $ | 1,068 | ​ | $ | 2,689,972 | ​ | $ | 405,935 | ​ | $ | 2,745,059 | ​ | $ | 407,003
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Held-to-Maturity Portfolio: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S Treasury and agency securities | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 1,015,549 | ​ | $ | 123,194 | ​ | $ | 1,015,549 | ​ | $ | 123,194
Government agency mortgage-backed securities | ​ | ​ | 149,742 | ​ | ​ | 2,027 | ​ | ​ | 15,281 | ​ | ​ | 214 | ​ | ​ | 165,023 | ​ | ​ | 2,241
Total held-to-maturity portfolio | ​ | $ | 149,742 | ​ | $ | 2,027 | ​ | $ | 1,030,830 | ​ | $ | 123,408 | ​ | $ | 1,180,572 | ​ | $ | 125,435
Schedule of amortized cost and estimated fair value of debt securities by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Held-to-Maturity | ​ | Available-for-Sale
​ | ​ | Amortized | ​ | Fair | ​ | Amortized | ​ | Fair
(000’s omitted) |  | Cost |  | Value |  | Cost |  | Value
Due in one year or less | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 16,195 | ​ | $ | 16,110
Due after one through five years | ​ |  | 0 | ​ |  | 0 | ​ | ​ | 1,948,062 | ​ | ​ | 1,836,112
Due after five years through ten years | ​ |  | 572,434 | ​ |  | 556,152 | ​ | ​ | 433,238 | ​ | ​ | 369,413
Due after ten years | ​ |  | 581,046 | ​ |  | 502,028 | ​ | ​ | 422,839 | ​ | ​ | 313,378
Subtotal | ​ |  | 1,153,480 | ​ |  | 1,058,180 | ​ | ​ | 2,820,334 | ​ | ​ | 2,535,013
Government agency mortgage-backed securities | ​ |  | 277,511 | ​ |  | 277,895 | ​ | ​ | 343,413 | ​ | ​ | 291,927
Government agency collateralized mortgage obligations | ​ |  | 0 | ​ |  | 0 | ​ | ​ | 5,685 | ​ | ​ | 5,430
Total | ​ | $ | 1,430,991 | ​ | $ | 1,336,075 | ​ | $ | 3,169,432 | ​ | $ | 2,832,370

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