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INVESTMENT SECURITIES (Tables) | 12 Months Ended
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Dec. 31, 2017
INVESTMENT SECURITIES [Abstract] |
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:
|  | 2017 |  |  | 2016 |
(000's omitted) |  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Estimated Fair Value |  |  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Estimated Fair Value |
Available-for-Sale Portfolio: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury and agency securities |  | $ | 2,043,023 |  |  | $ | 15,886 |  |  | $ | 4,838 |  |  | $ | 2,054,071 |  |  | $ | 1,876,358 |  |  | $ | 28,522 |  |  | $ | 2,118 |  |  | $ | 1,902,762 |
Obligations of state and political subdivisions |  |  | 514,949 |  |  |  | 14,064 |  |  |  | 57 |  |  |  | 528,956 |  |  |  | 582,655 |  |  |  | 13,389 |  |  |  | 1,054 |  |  |  | 594,990 |
Government agency mortgage-backed securities |  |  | 358,180 |  |  |  | 3,121 |  |  |  | 3,763 |  |  |  | 357,538 |  |  |  | 232,657 |  |  |  | 5,040 |  |  |  | 2,467 |  |  |  | 235,230 |
Corporate debt securities |  |  | 2,648 |  |  |  | 0 |  |  |  | 25 |  |  |  | 2,623 |  |  |  | 5,716 |  |  |  | 2 |  |  |  | 31 |  |  |  | 5,687 |
Government agency collateralized mortgage obligations |  |  | 88,097 |  |  |  | 155 |  |  |  | 878 |  |  |  | 87,374 |  |  |  | 9,225 |  |  |  | 310 |  |  |  | 0 |  |  |  | 9,535 |
Marketable equity securities |  |  | 251 |  |  |  | 275 |  |  |  | 0 |  |  |  | 526 |  |  |  | 252 |  |  |  | 200 |  |  |  | 0 |  |  |  | 452 |
Total available-for-sale portfolio |  | $ | 3,007,148 |  |  | $ | 33,501 |  |  | $ | 9,561 |  |  | $ | 3,031,088 |  |  | $ | 2,706,863 |  |  | $ | 47,463 |  |  | $ | 5,670 |  |  | $ | 2,748,656 |
Other Securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Federal Home Loan Bank common stock |  | $ | 9,896 |  |  |  |  |  |  |  |  |  |  | $ | 9,896 |  |  | $ | 12,191 |  |  |  |  |  |  |  |  |  |  | $ | 12,191 |
Federal Reserve Bank common stock |  |  | 30,690 |  |  |  |  |  |  |  |  |  |  |  | 30,690 |  |  |  | 19,781 |  |  |  |  |  |  |  |  |  |  |  | 19,781 |
Certificates of deposit |  |  | 3,865 |  |  |  |  |  |  |  |  |  |  |  | 3,865 |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  | 0 |
Other equity securities |  |  | 5,840 |  |  |  |  |  |  |  |  |  |  |  | 5,840 |  |  |  | 3,764 |  |  |  |  |  |  |  |  |  |  |  | 3,764 |
Total other securities |  | $ | 50,291 |  |  |  |  |  |  |  |  |  |  | $ | 50,291 |  |  | $ | 35,736 |  |  |  |  |  |  |  |  |  |  | $ | 35,736 |
Summary 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, 2017
|  |  |  |  | Less than 12 Months |  |  |  |  |  | 12 Months or Longer |  |  |  |  |  | Total |
(000's omitted) |  |  | # |  |  | Fair Value |  |  | Gross Unrealized Losses |  |  |  | # |  |  | Fair Value |  |  | Gross Unrealized Losses |  |  |  | # |  |  | Fair Value |  |  | Gross Unrealized Losses |
Available-for-Sale Portfolio: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury and agency securities |  |  | 44 |  |  | $ | 699,709 |  |  | $ | 4,838 |  |  |  | 0 |  |  | $ | 0 |  |  | $ | 0 |  |  |  | 44 |  |  | $ | 699,709 |  |  | $ | 4,838 |
Obligations of state and political subdivisions |  |  | 45 |  |  |  | 23,432 |  |  |  | 57 |  |  |  | 0 |  |  |  | 0 |  |  |  | 0 |  |  |  | 45 |  |  |  | 23,432 |  |  |  | 57 |
Government agency mortgage-backed securities |  |  | 120 |  |  |  | 185,716 |  |  |  | 1,433 |  |  |  | 55 |  |  |  | 75,712 |  |  |  | 2,330 |  |  |  | 175 |  |  |  | 261,428 |  |  |  | 3,763 |
Corporate debt securities |  |  | 1 |  |  |  | 2,623 |  |  |  | 25 |  |  |  | 0 |  |  |  | 0 |  |  |  | 0 |  |  |  | 1 |  |  |  | 2,623 |  |  |  | 25 |
Government agency collateralized mortgage obligations |  |  | 39 |  |  |  | 80,041 |  |  |  | 878 |  |  |  | 1 |  |  |  | 1 |  |  |  | 0 |  |  |  | 40 |  |  |  | 80,042 |  |  |  | 878 |
Total available-for-sale investment portfolio |  |  | 249 |  |  | $ | 991,521 |  |  | $ | 7,231 |  |  |  | 56 |  |  | $ | 75,713 |  |  | $ | 2,330 |  |  |  | 305 |  |  | $ | 1,067,234 |  |  | $ | 9,561 |
As of December 31, 2016
|  |  |  |  | Less than 12 Months |  |  |  |  |  | 12 Months or Longer |  |  |  |  |  | Total |
(000's omitted) |  |  | # |  |  | Fair Value |  |  | Gross Unrealized Losses |  |  |  | # |  |  | Fair Value |  |  | Gross Unrealized Losses |  |  |  | # |  |  | Fair Value |  |  | Gross Unrealized Losses |
Available-for-Sale Portfolio: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury and agency securities |  |  | 13 |  |  | $ | 449,242 |  |  | $ | 2,118 |  |  |  | 0 |  |  | $ | 0 |  |  | $ | 0 |  |  |  | 13 |  |  | $ | 449,242 |  |  | $ | 2,118 |
Obligations of state and political subdivisions |  |  | 197 |  |  |  | 102,106 |  |  |  | 1,054 |  |  |  | 0 |  |  |  | 0 |  |  |  | 0 |  |  |  | 197 |  |  |  | 102,106 |  |  |  | 1,054 |
Government agency mortgage-backed securities |  |  | 57 |  |  |  | 83,862 |  |  |  | 1,637 |  |  |  | 15 |  |  |  | 21,788 |  |  |  | 830 |  |  |  | 72 |  |  |  | 105,650 |  |  |  | 2,467 |
Corporate debt securities |  |  | 1 |  |  |  | 2,677 |  |  |  | 31 |  |  |  | 0 |  |  |  | 0 |  |  |  | 0 |  |  |  | 1 |  |  |  | 2,677 |  |  |  | 31 |
Government agency collateralized mortgage obligations |  |  | 0 |  |  |  | 0 |  |  |  | 0 |  |  |  | 2 |  |  |  | 2 |  |  |  | 0 |  |  |  | 2 |  |  |  | 2 |  |  |  | 0 |
Total available-for-sale investment portfolio |  |  | 268 |  |  | $ | 637,887 |  |  | $ | 4,840 |  |  |  | 17 |  |  | $ | 21,790 |  |  | $ | 830 |  |  |  | 285 |  |  | $ | 659,677 |  |  | $ | 5,670 |
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, 2017, 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 |
(000's omitted) |  | Amortized Cost |  |  | Fair Value |
Due in one year or less |  | $ | 45,966 |  |  | $ | 46,042 |
Due after one through five years |  |  | 1,565,423 |  |  |  | 1,572,634 |
Due after five years through ten years |  |  | 758,742 |  |  |  | 769,828 |
Due after ten years |  |  | 190,489 |  |  |  | 197,146 |
Subtotal |  |  | 2,560,620 |  |  |  | 2,585,650 |
Government agency mortgage-backed securities |  |  | 358,180 |  |  |  | 357,538 |
Government agency collateralized mortgage obligations |  |  | 88,097 |  |  |  | 87,374 |
Total |  | $ | 3,006,897 |  |  | $ | 3,030,562 |

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