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INVESTMENTS | 3 Months Ended
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Mar. 31, 2018
INVESTMENTS |
INVESTMENTS | 4.INVESTMENTS The following table summarizes the Company’s investments at:
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 |  | $ | - |
During the three-months ended March 31, 2018 and 2017, realized gains or losses recognized on the sale of investments were not significant. The Company’s investments at March 31, 2018 and December 31, 2017 in commercial paper, certificates of deposit, municipal securities, U.S. government agency securities and/or variable rate demand notes (“VRDNs”) carried investment grade credit ratings. VRDNs are floating rate municipal bonds with embedded put options that allow the bondholder to sell the security at par plus accrued interest. All of the put options are secured by a pledged liquidity source. While they are classified as marketable investment securities, the put option allows the VRDNs to be liquidated at par on a same day, or more generally, on a seven-day settlement basis. The following table summarizes the underlying contractual maturities of the Company’s investments at:
|  | 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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