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INVESTMENTS (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-sale |  |
Amortized Cost | $ 320,740 | $ 676,129
Gross Unrealized Holding Gains | 16 | 1
Gross Unrealized Holding Losses | 106 | 831
Fair Value | 320,650 | 675,299
Continuous Unrealized Loss Position less than 12 Months | 106 | 831
Short-term | Commercial paper |  |
Available-for-sale |  |
Amortized Cost | 52,838 | 81,026
Fair Value | 52,838 | 81,026
Short-term | Certificates of deposit |  |
Available-for-sale |  |
Amortized Cost | 14,075 | 11,869
Fair Value | 14,075 | 11,869
Short-term | Municipal securities |  |
Available-for-sale |  |
Amortized Cost | 151,690 | 469,604
Gross Unrealized Holding Gains | 16 | 1
Gross Unrealized Holding Losses | 62 | 740
Fair Value | 151,644 | 468,865
Continuous Unrealized Loss Position less than 12 Months | 62 | 740
Short-term | U.S. government agency securities |  |
Available-for-sale |  |
Amortized Cost | 19,943 | 61,307
Gross Unrealized Holding Losses | 12 | 88
Fair Value | 19,931 | 61,219
Continuous Unrealized Loss Position less than 12 Months | 12 | 88
Short-term | U.S. Treasuries |  |
Available-for-sale |  |
Amortized Cost | 78,189 |
Gross Unrealized Holding Losses | 32 |
Fair Value | 78,157 |
Continuous Unrealized Loss Position less than 12 Months | 32 |
Short-term | Variable rate demand notes |  |
Available-for-sale |  |
Amortized Cost | 4,005 | 49,954
Fair Value | $ 4,005 | 49,954
Long-term | U.S. government agency securities |  |
Available-for-sale |  |
Amortized Cost |  | 2,369
Gross Unrealized Holding Losses |  | 3
Fair Value |  | 2,366
Continuous Unrealized Loss Position less than 12 Months |  | $ 3

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