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MARKETABLE SECURITIES (Details) Securities in Thousands, $ in Thousands | Dec. 31, 2019 USD ($) Securities | Dec. 31, 2018 USD ($) Securities
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 231,815 | $ 94,717
Gross Unrealized Gains | 589 | 2
Gross Unrealized Losses | (6) | (268)
Estimated Fair Market Value | $ 232,398 | $ 94,451
Weighted Average Interest Rate on Investments | 2.17% | 2.65%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | Securities | 0 | 0
Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 15,934 | $ 6,788
Gross Unrealized Gains | 18 | 0
Gross Unrealized Losses | 0 | (2)
Estimated Fair Market Value | 15,952 | 6,786
Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 71,223 | 60,123
Gross Unrealized Gains | 269 | 0
Gross Unrealized Losses | 0 | (244)
Estimated Fair Market Value | 71,492 | 59,879
Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 144,658 | 27,806
Gross Unrealized Gains | 302 | 2
Gross Unrealized Losses | (6) | (22)
Estimated Fair Market Value | 144,954 | 27,786
Corporate securities | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 15,934 | 6,788
Gross Unrealized Gains | 18 | 0
Gross Unrealized Losses | 0 | (2)
Estimated Fair Market Value | 15,952 | 6,786
Corporate securities | Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 71,223 | 60,123
Gross Unrealized Gains | 269 | 0
Gross Unrealized Losses | 0 | (244)
Estimated Fair Market Value | 71,492 | 59,879
Corporate securities | Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 144,658 | 27,806
Gross Unrealized Gains | 302 | 2
Gross Unrealized Losses | (6) | (22)
Estimated Fair Market Value | $ 144,954 | $ 27,786

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