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INVESTMENT SECURITIES (Tables) | 12 Months Ended
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Dec. 31, 2023
INVESTMENT SECURITIES |
Schedule of amortized cost and estimated fair value of investment securities | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2023 |  | 2022
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Estimated | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Estimated
(000’s omitted) |  | Cost |  | Gains |  | Losses |  | Fair Value |  | Cost |  | Gains |  | Losses |  | Fair Value
Available-for-Sale Portfolio: | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​
U.S. Treasury and agency securities | ​ | $ | 2,381,168 | ​ | $ | 0 | ​ | $ | 300,385 | ​ | $ | 2,080,783 | ​ | $ | 3,660,546 | ​ | $ | 0 | ​ | $ | 417,009 | ​ | $ | 3,243,537
Obligations of state and political subdivisions | ​ | ​ | 502,879 | ​ | ​ | 1,469 | ​ | ​ | 29,985 | ​ | ​ | 474,363 | ​ | ​ | 549,118 | ​ | ​ | 506 | ​ | ​ | 45,327 | ​ | ​ | 504,297
Government agency mortgage-backed securities | ​ | ​ | 400,062 | ​ | ​ | 76 | ​ | ​ | 51,612 | ​ | ​ | 348,526 | ​ | ​ | 444,689 | ​ | ​ | 58 | ​ | ​ | 60,114 | ​ | ​ | 384,633
Corporate debt securities | ​ | ​ | 8,000 | ​ | ​ | 0 | ​ | ​ | 606 | ​ | ​ | 7,394 | ​ | ​ | 8,000 | ​ | ​ | 0 | ​ | ​ | 886 | ​ | ​ | 7,114
Government agency collateralized mortgage obligations | ​ | ​ | 9,498 | ​ | ​ | 0 | ​ | ​ | 572 | ​ | ​ | 8,926 | ​ | ​ | 13,121 | ​ | ​ | 1 | ​ | ​ | 852 | ​ | ​ | 12,270
Total available-for-sale portfolio | ​ | $ | 3,301,607 | ​ | $ | 1,545 | ​ | $ | 383,160 | ​ | $ | 2,919,992 | ​ | $ | 4,675,474 | ​ | $ | 565 | ​ | $ | 524,188 | ​ | $ | 4,151,851
Held-to-Maturity Portfolio: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury and agency securities | ​ | $ | 1,109,101 | ​ | $ | 0 | ​ | $ | 50,866 | ​ | $ | 1,058,235 | ​ | $ | 1,079,695 | ​ | $ | 0 | ​ | $ | 44,900 | ​ | $ | 1,034,795
Government agency mortgage-backed securities | ​ | ​ | 63,073 | ​ | ​ | 688 | ​ | ​ | 180 | ​ | ​ | 63,581 | ​ | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 0
Total held-to-maturity portfolio | ​ | $ | 1,172,174 | ​ | $ | 688 | ​ | $ | 51,046 | ​ | $ | 1,121,816 | ​ | $ | 1,079,695 | ​ | $ | 0 | ​ | $ | 44,900 | ​ | $ | 1,034,795
Equity and other Securities: | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​
Equity securities, at fair value | ​ | $ | 251 | ​ | $ | 121 | ​ | $ | 0 | ​ | $ | 372 | ​ | $ | 251 | ​ | $ | 168 | ​ | $ | 0 | ​ | $ | 419
Federal Home Loan Bank common stock | ​ |  | 32,526 | ​ |  | 0 | ​ |  | 0 | ​ |  | 32,526 | ​ |  | 47,497 | ​ |  | 0 | ​ |  | 0 | ​ |  | 47,497
Federal Reserve Bank common stock | ​ |  | 33,568 | ​ |  | 0 | ​ |  | 0 | ​ |  | 33,568 | ​ |  | 31,144 | ​ |  | 0 | ​ |  | 0 | ​ |  | 31,144
Other equity securities, at adjusted cost | ​ |  | 5,930 | ​ |  | 750 | ​ |  | 0 | ​ |  | 6,680 | ​ |  | 3,532 | ​ |  | 750 | ​ |  | 0 | ​ |  | 4,282
Total equity and other securities | ​ | $ | 72,275 | ​ | $ | 871 | ​ | $ | 0 | ​ | $ | 73,146 | ​ | $ | 82,424 | ​ | $ | 918 | ​ | $ | 0 | ​ | $ | 83,342
Schedule of investment securities that have been in a continuous unrealized loss position | As of December 31, 2023 ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | 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 | ​ | $ | 2,080,783 | ​ | $ | 300,385 | ​ | $ | 2,080,783 | ​ | $ | 300,385
Obligations of state and political subdivisions | ​ |  | 63,541 | ​ |  | 878 | ​ |  | 287,191 | ​ |  | 29,107 | ​ |  | 350,732 | ​ |  | 29,985
Government agency mortgage-backed securities | ​ |  | 8,586 | ​ |  | 55 | ​ |  | 336,266 | ​ |  | 51,557 | ​ |  | 344,852 | ​ |  | 51,612
Corporate debt securities | ​ | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 7,394 | ​ | ​ | 606 | ​ | ​ | 7,394 | ​ | ​ | 606
Government agency collateralized mortgage obligations | ​ |  | 0 | ​ |  | 0 | ​ |  | 8,907 | ​ |  | 572 | ​ |  | 8,907 | ​ |  | 572
Total available-for-sale investment portfolio | ​ | $ | 72,127 | ​ | $ | 933 | ​ | $ | 2,720,541 | ​ | $ | 382,227 | ​ | $ | 2,792,668 | ​ | $ | 383,160
Held-to-Maturity Portfolio: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S Treasury and agency securities | ​ | $ | 536,885 | ​ | $ | 15,953 | ​ | $ | 521,350 | ​ | $ | 34,913 | ​ | $ | 1,058,235 | ​ | $ | 50,866
Government agency mortgage-backed securities | ​ | ​ | 18,951 | ​ | ​ | 158 | ​ | ​ | 1,393 | ​ | ​ | 22 | ​ | ​ | 20,344 | ​ | ​ | 180
Total held-to-maturity portfolio | ​ | $ | 555,836 | ​ | $ | 16,111 | ​ | $ | 522,743 | ​ | $ | 34,935 | ​ | $ | 1,078,579 | ​ | $ | 51,046
​ As of December 31, 2022 ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | 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 | ​ | $ | 1,384,075 | ​ | $ | 132,511 | ​ | $ | 1,859,462 | ​ | $ | 284,498 | ​ | $ | 3,243,537 | ​ | $ | 417,009
Obligations of state and political subdivisions | ​ |  | 370,524 | ​ |  | 35,488 | ​ |  | 47,923 | ​ |  | 9,839 | ​ |  | 418,447 | ​ |  | 45,327
Government agency mortgage-backed securities | ​ |  | 190,727 | ​ |  | 19,508 | ​ |  | 189,919 | ​ |  | 40,606 | ​ |  | 380,646 | ​ |  | 60,114
Corporate debt securities | ​ | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 7,114 | ​ | ​ | 886 | ​ | ​ | 7,114 | ​ | ​ | 886
Government agency collateralized mortgage obligations | ​ |  | 9,968 | ​ |  | 600 | ​ |  | 2,274 | ​ |  | 252 | ​ |  | 12,242 | ​ |  | 852
Total available-for-sale investment portfolio | ​ | $ | 1,955,294 | ​ | $ | 188,107 | ​ | $ | 2,106,692 | ​ | $ | 336,081 | ​ | $ | 4,061,986 | ​ | $ | 524,188
Held-to-Maturity Portfolio: | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  |  | ​ |  | ​
U.S Treasury and agency securities | ​ | $ | 1,034,795 | ​ | $ | 44,900 | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 1,034,795 | ​ | $ | 44,900
Total held-to-maturity portfolio | ​ | $ | 1,034,795 | ​ | $ | 44,900 | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 1,034,795 | ​ | $ | 44,900
Schedule of amortized cost and estimated fair value of debt securities by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Held-to-Maturity |  | Available-for-Sale
(000’s omitted) | ​ | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value
Due in one year or less | ​ | $ | 0 | ​ | $ | 0 | ​ | $ | 16,671 | ​ | $ | 16,566
Due after one through five years | ​ | ​ | 0 | ​ | ​ | 0 | ​ |  | 1,583,809 | ​ |  | 1,441,328
Due after five years through ten years | ​ | ​ | 552,838 | ​ | ​ | 536,885 | ​ |  | 579,092 | ​ |  | 523,830
Due after ten years | ​ | ​ | 556,263 | ​ | ​ | 521,350 | ​ |  | 712,475 | ​ |  | 580,816
Subtotal | ​ | ​ | 1,109,101 | ​ | ​ | 1,058,235 | ​ |  | 2,892,047 | ​ |  | 2,562,540
Government agency mortgage-backed securities | ​ | ​ | 63,073 | ​ | ​ | 63,581 | ​ |  | 400,062 | ​ |  | 348,526
Government agency collateralized mortgage obligations | ​ | ​ | 0 | ​ | ​ | 0 | ​ |  | 9,498 | ​ |  | 8,926
Total | ​ | $ | 1,172,174 | ​ | $ | 1,121,816 | ​ | $ | 3,301,607 | ​ | $ | 2,919,992

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