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INVESTMENTS (Tables) | 9 Months Ended
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Sep. 30, 2018
INVESTMENTS |
Summary of investments in available-for-sale |
September 30, 2018 |  | Amortized Cost |  | Gross Unrealized Holding Gains |  | Gross Unrealized Holding Losses |  | Fair Value |  | Continuous Unrealized Loss Position less than 12 Months |  | Continuous Unrealized Loss Position greater than 12 Months |
Available-for-sale |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term: |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 52,595 |  | $ | - |  | $ | - |  | $ | 52,595 |  | $ | - |  | $ | - |
Certificates of deposit |  | 26,068 |  | - |  | - |  | 26,068 |  | - |  | - |
Municipal securities |  | 127,149 |  | 2 |  | 82 |  | 127,069 |  | 82 |  | - |
U.S. government agency securities |  | 108,128 |  | - |  | 95 |  | 108,033 |  | 95 |  | - |
U.S. Treasuries |  | 109,832 |  | - |  | 45 |  | 109,787 |  | 45 |  | - |
Variable rate demand notes |  | 34,346 |  | - |  | - |  | 34,346 |  | - |  | - |
Long-term: |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal securities |  | 1,612 |  | - |  | 2 |  | 1,610 |  | 2 |  | - |
Total |  | $ | 459,730 |  | $ | 2 |  | $ | 224 |  | $ | 459,508 |  | $ | 224 |  | $ | - |
December 31, 2017 |  | Amortized Cost |  | Gross Unrealized Holding Gains |  | Gross Unrealized Holding Losses |  | Fair Value |  | Continuous Unrealized Loss Position less than 12 Months |  | Continuous Unrealized Loss Position greater than 12 Months |
Available-for-sale |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term: |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 81,026 |  | $ | - |  | $ | - |  | $ | 81,026 |  | $ | - |  | $ | - |
Certificates of deposit |  | 11,869 |  | - |  | - |  | 11,869 |  | - |  | - |
Municipal securities |  | 469,604 |  | 1 |  | 740 |  | 468,865 |  | 740 |  | - |
U.S. government agency securities |  | 61,307 |  | - |  | 88 |  | 61,219 |  | 88 |  | - |
Variable rate demand notes |  | 49,954 |  | - |  | - |  | 49,954 |  | - |  | - |
Long-term: |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. government agency securities |  | 2,369 |  | - |  | 3 |  | 2,366 |  | 3 |  | - |
Total |  | $ | 676,129 |  | $ | 1 |  | $ | 831 |  | $ | 675,299 |  | $ | 831 |  | $ | - |
Summary of the underlying contractual maturities of investments |
|  | September 30, 2018 |  | December 31, 2017 |
|  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value |
Less than 1 year: |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 52,595 |  | $ | 52,595 |  | $ | 81,026 |  | $ | 81,026 |
U.S. Treasuries |  | 109,832 |  | 109,787 |  | - |  | - |
Municipal securities |  | 127,149 |  | 127,069 |  | 469,604 |  | 468,865 |
U.S. government agency securities |  | 108,128 |  | 108,033 |  | 61,307 |  | 61,219 |
Certificates of deposit |  | 26,068 |  | 26,068 |  | 11,869 |  | 11,869 |
Due 1 - 10 years: |  |  |  |  |  |  |  |  |
Municipal securities |  | 1,612 |  | 1,610 |  | - |  | - |
U.S. government agency securities |  | - |  | - |  | 2,369 |  | 2,366 |
Variable rate demand notes |  | - |  | - |  | 6,366 |  | 6,366 |
Due 11 - 20 years: |  |  |  |  |  |  |  |  |
Variable rate demand notes |  | 15,952 |  | 15,952 |  | 28,377 |  | 28,377 |
Due 21 - 30 years: |  |  |  |  |  |  |  |  |
Variable rate demand notes |  | 14,291 |  | 14,291 |  | 15,211 |  | 15,211 |
Due 31 - 40 years: |  |  |  |  |  |  |  |  |
Variable rate demand notes |  | 4,103 |  | 4,103 |  | - |  | - |
Total |  | $ | 459,730 |  | $ | 459,508 |  | $ | 676,129 |  | $ | 675,299 |

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