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MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Interest receivable | $ 1,712 | $ 1,279
Amortized Cost | 232,552 | 231,815
Gross Unrealized Gains | 207 | 589
Gross Unrealized Losses | (576) | (6)
Estimated Fair Market Value | 232,183 | 232,398
Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 42,973 | 15,934
Gross Unrealized Gains | 1 | 18
Gross Unrealized Losses | (21) | 0
Estimated Fair Market Value | 42,953 | 15,952
Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 63,204 | 71,223
Gross Unrealized Gains | 61 | 269
Gross Unrealized Losses | (159) | 0
Estimated Fair Market Value | 63,106 | 71,492
Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 126,375 | 144,658
Gross Unrealized Gains | 145 | 302
Gross Unrealized Losses | (396) | (6)
Estimated Fair Market Value | 126,124 | 144,954
Commercial paper | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 14,869 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Estimated Fair Market Value | 14,869 |
Corporate securities | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 28,104 | 15,934
Gross Unrealized Gains | 1 | 18
Gross Unrealized Losses | (21) | 0
Estimated Fair Market Value | 28,084 | 15,952
Corporate securities | Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 63,204 | 71,223
Gross Unrealized Gains | 61 | 269
Gross Unrealized Losses | (159) | 0
Estimated Fair Market Value | 63,106 | 71,492
Corporate securities | Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 126,375 | 144,658
Gross Unrealized Gains | 145 | 302
Gross Unrealized Losses | (396) | 0
Estimated Fair Market Value | $ 126,124 | $ 144,954

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