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INVESTMENTS (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Available-for-sale |  |
Amortized Cost | $ 925,559 | $ 545,771
Gross Unrealized Holding Gains | 165 | 285
Gross Unrealized Holding Losses | 79 | 88
Fair Value | 925,645 | 545,968
Continuous Unrealized Loss Position less than 12 Months | 79 | 88
Short-term | Commercial paper |  |
Available-for-sale |  |
Amortized Cost | 119,886 | 83,478
Fair Value | 119,886 | 83,478
Short-term | Certificates of deposit |  |
Available-for-sale |  |
Amortized Cost | 20,387 | 28,049
Fair Value | 20,387 | 28,049
Short-term | Municipal securities |  |
Available-for-sale |  |
Amortized Cost | 9,083 | 147,983
Gross Unrealized Holding Gains |  | 145
Gross Unrealized Holding Losses |  | 20
Fair Value | 9,083 | 148,108
Continuous Unrealized Loss Position less than 12 Months |  | 20
Short-term | U.S. government agency securities |  |
Available-for-sale |  |
Amortized Cost | 81,521 | 40,620
Gross Unrealized Holding Gains | 13 | 5
Gross Unrealized Holding Losses | 3 | 35
Fair Value | 81,531 | 40,590
Continuous Unrealized Loss Position less than 12 Months | 3 | 35
Short-term | U.S. treasuries |  |
Available-for-sale |  |
Amortized Cost | 650,386 | 211,055
Gross Unrealized Holding Gains | 150 | 134
Gross Unrealized Holding Losses | 69 | 31
Fair Value | 650,467 | 211,158
Continuous Unrealized Loss Position less than 12 Months | 69 | 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 | 10,350 | 5,267
Gross Unrealized Holding Gains | 1 |
Gross Unrealized Holding Losses |  | 1
Fair Value | 10,351 | 5,266
Continuous Unrealized Loss Position less than 12 Months |  | 1
Long-term | U.S. treasuries |  |
Available-for-sale |  |
Amortized Cost | 33,946 | 6,077
Gross Unrealized Holding Gains | 1 | 1
Gross Unrealized Holding Losses | 7 |
Fair Value | 33,940 | $ 6,078
Continuous Unrealized Loss Position less than 12 Months | $ 7 |

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