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MARKETABLE SECURITIES (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 189,428 | $ 231,815
Gross Unrealized Gains | 890 | 589
Gross Unrealized Losses | 0 | (6)
Estimated Fair Market Value | $ 190,318 | $ 232,398
Weighted Average Interest Rate on Investments | 0.89% | 2.17%
Interest receivable | $ 800 | $ 1,300
Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 63,506 | 15,934
Gross Unrealized Gains | 44 | 18
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 63,550 | 15,952
Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 125,922 | 71,223
Gross Unrealized Gains | 846 | 269
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 126,768 | 71,492
Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 144,658
Gross Unrealized Gains |  | 302
Gross Unrealized Losses |  | (6)
Estimated Fair Market Value |  | 144,954
Commercial paper | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 43,660 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Estimated Fair Market Value | 43,660 |
Corporate securities | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 19,846 | 15,934
Gross Unrealized Gains | 44 | 18
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 19,890 | 15,952
Corporate securities | Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 125,922 | 71,223
Gross Unrealized Gains | 846 | 269
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | $ 126,768 | 71,492
Corporate securities | Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 144,658
Gross Unrealized Gains |  | 302
Gross Unrealized Losses |  | (6)
Estimated Fair Market Value |  | $ 144,954

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