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INVESTMENT SECURITIES (Tables) | 12 Months Ended
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Dec. 31, 2021
INVESTMENT SECURITIES. |
Schedule of amortized cost and estimated fair value of investment securities | The amortized cost and estimated fair value of investment securities as of December 31 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2021 |  | 2020
​ | ​ | ​ | ​ | ​ | 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 | ​ | $ | 4,064,624 | ​ | $ | 39,997 | ​ | $ | 106,057 | ​ | $ | 3,998,564 | ​ | $ | 2,423,236 | ​ | $ | 94,741 | ​ | $ | 16,595 | ​ | $ | 2,501,382
Obligations of state and political subdivisions | ​ | ​ | 413,019 | ​ | ​ | 17,326 | ​ | ​ | 56 | ​ | ​ | 430,289 | ​ | ​ | 451,028 | ​ | ​ | 24,632 | ​ | ​ | 0 | ​ | ​ | 475,660
Government agency mortgage-backed securities | ​ | ​ | 474,506 | ​ | ​ | 7,615 | ​ | ​ | 5,065 | ​ | ​ | 477,056 | ​ | ​ | 506,540 | ​ | ​ | 16,280 | ​ | ​ | 182 | ​ | ​ | 522,638
Corporate debt securities | ​ | ​ | 8,000 | ​ | ​ | 39 | ​ | ​ | 77 | ​ | ​ | 7,962 | ​ | ​ | 4,499 | ​ | ​ | 137 | ​ | ​ | 1 | ​ | ​ | 4,635
Government agency collateralized mortgage obligations | ​ | ​ | 19,953 | ​ | ​ | 410 | ​ | ​ | 24 | ​ | ​ | 20,339 | ​ | ​ | 42,476 | ​ | ​ | 1,111 | ​ | ​ | 10 | ​ | ​ | 43,577
Total available-for-sale portfolio | ​ | $ | 4,980,102 | ​ | $ | 65,387 | ​ | $ | 111,279 | ​ | $ | 4,934,210 | ​ | $ | 3,427,779 | ​ | $ | 136,901 | ​ | $ | 16,788 | ​ | $ | 3,547,892
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Equity and other Securities: | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​ | ​ |  | ​
Equity securities, at fair value | ​ | $ | 251 | ​ | $ | 212 | ​ | $ | 0 | ​ | $ | 463 | ​ | $ | 251 | ​ | $ | 194 | ​ | $ | 0 | ​ | $ | 445
Federal Home Loan Bank common stock | ​ |  | 7,188 | ​ |  | 0 | ​ |  | 0 | ​ |  | 7,188 | ​ |  | 7,468 | ​ |  | 0 | ​ |  | 0 | ​ |  | 7,468
Federal Reserve Bank common stock | ​ |  | 33,916 | ​ |  | 0 | ​ |  | 0 | ​ |  | 33,916 | ​ |  | 33,916 | ​ |  | 0 | ​ |  | 0 | ​ |  | 33,916
Other equity securities, at adjusted cost | ​ |  | 2,562 | ​ |  | 750 | ​ |  | 0 | ​ |  | 3,312 | ​ |  | 4,876 | ​ |  | 750 | ​ |  | 0 | ​ |  | 5,626
Total equity and other securities | ​ | $ | 43,917 | ​ | $ | 962 | ​ | $ | 0 | ​ | $ | 44,879 | ​ | $ | 46,511 | ​ | $ | 944 | ​ | $ | 0 | ​ | $ | 47,455
​
Schedule of investment securities that have been in a continuous unrealized loss position for less than or greater than twelve months | A summary of investment securities that have been in a continuous unrealized loss position for less than or greater than twelve months is as follows: As of December 31, 2021 ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 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 | ​ | 47 | ​ | $ | 1,224,101 | ​ | $ | 14,873 | ​ | 13 | ​ | $ | 900,462 | ​ | $ | 91,184 | ​ | 60 | ​ | $ | 2,124,563 | ​ | $ | 106,057
Obligations of state and political subdivisions | ​ | 27 | ​ |  | 23,966 | ​ |  | 56 | ​ | 0 | ​ |  | 0 | ​ |  | 0 | ​ | 27 | ​ |  | 23,966 | ​ |  | 56
Government agency mortgage-backed securities | ​ | 147 | ​ |  | 139,442 | ​ |  | 2,475 | ​ | 52 | ​ |  | 67,273 | ​ |  | 2,590 | ​ | 199 | ​ |  | 206,715 | ​ |  | 5,065
Corporate debt securities | ​ | 1 | ​ | ​ | 4,923 | ​ | ​ | 77 | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 0 | ​ | 1 | ​ | ​ | 4,923 | ​ | ​ | 77
Government agency collateralized mortgage obligations | ​ | 18 | ​ |  | 3,146 | ​ |  | 24 | ​ | 1 | ​ |  | 53 | ​ |  | 0 | ​ | 19 | ​ |  | 3,199 | ​ |  | 24
Total available-for-sale investment portfolio | ​ | 240 | ​ | $ | 1,395,578 | ​ | $ | 17,505 | ​ | 66 | ​ | $ | 967,788 | ​ | $ | 93,774 | ​ | 306 | ​ | $ | 2,363,366 | ​ | $ | 111,279
​ As of December 31, 2020 ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 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 | ​ | 13 | ​ | $ | 831,015 | ​ | $ | 16,595 | ​ | 0 | ​ | $ | 0 | ​ | $ | 0 | ​ | 13 | ​ | $ | 831,015 | ​ | $ | 16,595
Obligations of state and political subdivisions | ​ | 1 | ​ |  | 358 | ​ |  | 0 | ​ | 0 | ​ |  | 0 | ​ |  | 0 | ​ | 1 | ​ |  | 358 | ​ |  | 0
Government agency mortgage-backed securities | ​ | 89 | ​ |  | 75,992 | ​ |  | 182 | ​ | 2 | ​ |  | 14 | ​ |  | 0 | ​ | 91 | ​ |  | 76,006 | ​ |  | 182
Corporate debt securities | ​ | 1 | ​ | ​ | 1,001 | ​ | ​ | 1 | ​ | 0 | ​ | ​ | 0 | ​ | ​ | 0 | ​ | 1 | ​ | ​ | 1,001 | ​ | ​ | 1
Government agency collateralized mortgage obligations | ​ | 13 | ​ |  | 5,246 | ​ |  | 10 | ​ | 1 | ​ |  | 0 | ​ |  | 0 | ​ | 14 | ​ |  | 5,246 | ​ |  | 10
Total available-for-sale investment portfolio | ​ | 117 | ​ | $ | 913,612 | ​ | $ | 16,788 | ​ | 3 | ​ | $ | 14 | ​ | $ | 0 | ​ | 120 | ​ | $ | 913,626 | ​ | $ | 16,788
​
Schedule of amortized cost and estimated fair value of debt securities by contractual maturity | The amortized cost and estimated fair value of debt securities at December 31, 2021, by contractual maturity, are shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. Securities not due at a single maturity date are shown separately. ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Available-for-Sale
​ | ​ | Amortized | ​ | ​
(000’s omitted) |  | Cost |  | Fair Value
Due in one year or less | ​ | $ | 156,262 | ​ | $ | 158,119
Due after one through five years | ​ |  | 876,849 | ​ |  | 891,772
Due after five years through ten years | ​ |  | 1,958,202 | ​ |  | 1,962,453
Due after ten years | ​ |  | 1,494,330 | ​ |  | 1,424,471
Subtotal | ​ |  | 4,485,643 | ​ |  | 4,436,815
Government agency mortgage-backed securities | ​ |  | 474,506 | ​ |  | 477,056
Government agency collateralized mortgage obligations | ​ |  | 19,953 | ​ |  | 20,339
Total | ​ | $ | 4,980,102 | ​ | $ | 4,934,210
​

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