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INVESTMENTS (Tables) | 3 Months Ended
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Mar. 31, 2018
INVESTMENTS |
Summary of investments in available-for-sale |
March 31, 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 |  | $ | 42,288 |  | $ | - |  | $ | - |  | $ | 42,288 |  | $ | - |  | $ | - |
Municipal securities |  | 441,360 |  | - |  | 508 |  | 440,852 |  | 508 |  | - |
U.S. government agency securities |  | 74,773 |  | - |  | 107 |  | 74,666 |  | 107 |  | - |
Variable rate demand notes |  | 27,416 |  | - |  | - |  | 27,416 |  | - |  | - |
Long-term: |  |  |  |  |  |  |  |  |  |  |  |  |
Municipal securities |  | 527 |  | - |  | - |  | 527 |  | - |  | - |
U.S. government agency securities |  | 2,973 |  | - |  | 3 |  | 2,970 |  | 3 |  | - |
Total |  | $ | 589,337 |  | $ | - |  | $ | 618 |  | $ | 588,719 |  | $ | 618 |  | $ | - |
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 |  | $ | - |
Summarizes the underlying contractual maturities of the Company's investments |
|  | March 31, 2018 |  | December 31, 2017 |
|  | Amortized Cost |  | Fair Value |  | Amortized Cost |  | Fair Value |
Less than 1 year: |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 42,288 |  | $ | 42,288 |  | $ | 81,026 |  | $ | 81,026 |
Municipal securities |  | 441,360 |  | 440,852 |  | 469,604 |  | 468,865 |
U.S. government agency securities |  | 74,773 |  | 74,666 |  | 61,307 |  | 61,219 |
Certificates of deposit |  | - |  | - |  | 11,869 |  | 11,869 |
Due 1 -10 years: |  |  |  |  |  |  |  |  |
Municipal securities |  | 527 |  | 527 |  | - |  | - |
U.S. government agency securities |  | 2,973 |  | 2,970 |  | 2,369 |  | 2,366 |
Variable rate demand notes |  | 6,622 |  | 6,622 |  | 6,366 |  | 6,366 |
Due 11 - 20 years: |  |  |  |  |  |  |  |  |
Variable rate demand notes |  | 18,542 |  | 18,542 |  | 28,377 |  | 28,377 |
Due 21 - 30 years: |  |  |  |  |  |  |  |  |
Variable rate demand notes |  | 2,252 |  | 2,252 |  | 15,211 |  | 15,211 |
Total |  | $ | 589,337 |  | $ | 588,719 |  | $ | 676,129 |  | $ | 675,299 |

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