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MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Interest receivable | $ 1,395 | $ 1,279
Amortized Cost | 210,362 | 231,815
Gross Unrealized Gains | 1,564 | 589
Gross Unrealized Losses | 0 | (6)
Estimated Fair Market Value | 211,926 | 232,398
Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 64,370 | 15,934
Gross Unrealized Gains | 52 | 18
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 64,422 | 15,952
Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 136,410 | 71,223
Gross Unrealized Gains | 1,366 | 269
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 137,776 | 71,492
Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 9,582 | 144,658
Gross Unrealized Gains | 146 | 302
Gross Unrealized Losses | 0 | (6)
Estimated Fair Market Value | 9,728 | 144,954
Commercial paper | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 46,251 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Estimated Fair Market Value | 46,251 |
Corporate securities | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 18,119 | 15,934
Gross Unrealized Gains | 52 | 18
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 18,171 | 15,952
Corporate securities | Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 136,410 | 71,223
Gross Unrealized Gains | 1,366 | 269
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 137,776 | 71,492
Corporate securities | Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 9,582 | 144,658
Gross Unrealized Gains | 146 | 302
Gross Unrealized Losses | 0 | (6)
Estimated Fair Market Value | $ 9,728 | $ 144,954

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