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INVESTMENTS (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Available-for-sale |  |
Amortized Cost | $ 459,730 | $ 676,129
Gross Unrealized Holding Gains | 2 | 1
Gross Unrealized Holding Losses | 224 | 831
Fair Value | 459,508 | 675,299
Continuous Unrealized Loss Position less than 12 Months | 224 | 831
Short-term | Commercial paper |  |
Available-for-sale |  |
Amortized Cost | 52,595 | 81,026
Fair Value | 52,595 | 81,026
Short-term | Certificates of deposit |  |
Available-for-sale |  |
Amortized Cost | 26,068 | 11,869
Fair Value | 26,068 | 11,869
Short-term | Municipal securities |  |
Available-for-sale |  |
Amortized Cost | 127,149 | 469,604
Gross Unrealized Holding Gains | 2 | 1
Gross Unrealized Holding Losses | 82 | 740
Fair Value | 127,069 | 468,865
Continuous Unrealized Loss Position less than 12 Months | 82 | 740
Short-term | U.S. government agency securities |  |
Available-for-sale |  |
Amortized Cost | 108,128 | 61,307
Gross Unrealized Holding Losses | 95 | 88
Fair Value | 108,033 | 61,219
Continuous Unrealized Loss Position less than 12 Months | 95 | 88
Short-term | U.S. Treasuries |  |
Available-for-sale |  |
Amortized Cost | 109,832 |
Gross Unrealized Holding Losses | 45 |
Fair Value | 109,787 |
Continuous Unrealized Loss Position less than 12 Months | 45 |
Short-term | Variable rate demand notes |  |
Available-for-sale |  |
Amortized Cost | 34,346 | 49,954
Fair Value | 34,346 | 49,954
Long-term | Municipal securities |  |
Available-for-sale |  |
Amortized Cost | 1,612 |
Gross Unrealized Holding Losses | 2 |
Fair Value | 1,610 |
Continuous Unrealized Loss Position less than 12 Months | $ 2 |
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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