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INVESTMENTS (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Available-for-sale |  |
Amortized Cost | $ 252,173 | $ 545,771
Gross Unrealized Holding Gains | 674 | 285
Gross Unrealized Holding Losses | 17 | 88
Fair Value | 252,830 | 545,968
Continuous Unrealized Loss Position less than 12 Months | 17 | 88
Short-term | Commercial paper |  |
Available-for-sale |  |
Amortized Cost | 58,540 | 83,478
Fair Value | 58,540 | 83,478
Short-term | Certificates of deposit |  |
Available-for-sale |  |
Amortized Cost | 3,624 | 28,049
Fair Value | 3,624 | 28,049
Short-term | Municipal securities |  |
Available-for-sale |  |
Amortized Cost |  | 147,983
Gross Unrealized Holding Gains |  | 145
Gross Unrealized Holding Losses |  | 20
Fair Value |  | 148,108
Continuous Unrealized Loss Position less than 12 Months |  | 20
Short-term | U.S. government agency securities |  |
Available-for-sale |  |
Amortized Cost | 44,851 | 40,620
Gross Unrealized Holding Gains | 41 | 5
Gross Unrealized Holding Losses | 9 | 35
Fair Value | 44,883 | 40,590
Continuous Unrealized Loss Position less than 12 Months | 9 | 35
Short-term | U.S. treasuries |  |
Available-for-sale |  |
Amortized Cost | 143,081 | 211,055
Gross Unrealized Holding Gains | 633 | 134
Gross Unrealized Holding Losses | 8 | 31
Fair Value | 143,706 | 211,158
Continuous Unrealized Loss Position less than 12 Months | 8 | 31
Short-term | Variable rate demand notes |  |
Available-for-sale |  |
Amortized Cost |  | 21,680
Fair Value |  | 21,680
Long-term | Municipal securities |  |
Available-for-sale |  |
Amortized Cost |  | 1,562
Gross Unrealized Holding Losses |  | 1
Fair Value |  | 1,561
Continuous Unrealized Loss Position less than 12 Months |  | 1
Long-term | U.S. government agency securities |  |
Available-for-sale |  |
Amortized Cost |  | 5,267
Gross Unrealized Holding Losses |  | 1
Fair Value |  | 5,266
Continuous Unrealized Loss Position less than 12 Months |  | 1
Long-term | U.S. treasuries |  |
Available-for-sale |  |
Amortized Cost | 2,077 | 6,077
Gross Unrealized Holding Gains |  | 1
Fair Value | $ 2,077 | $ 6,078

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