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MARKETABLE SECURITIES (Details) Securities in Thousands, $ in Thousands | Dec. 31, 2018 USD ($) Securities | Dec. 31, 2017 USD ($) Securities
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 94,717 | $ 189,663
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | (268) | (427)
Estimated Fair Market Value | $ 94,451 | $ 189,236
Weighted Average Interest Rate on Investments | 2.65% | 1.57%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | Securities | 0 |
Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 6,788 | $ 38,485
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (2) | (16)
Estimated Fair Market Value | 6,786 | 38,469
Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 60,123 | 113,742
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (244) | (216)
Estimated Fair Market Value | 59,879 | 113,526
Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 27,806 | 37,436
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | (22) | (195)
Estimated Fair Market Value | 27,786 | $ 37,241
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | Securities |  | 0
Corporate securities | Investments due in 3 months or less: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 6,788 | $ 38,485
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (2) | (16)
Estimated Fair Market Value | 6,786 | 38,469
Corporate securities | Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 60,123 | 104,440
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (244) | (199)
Estimated Fair Market Value | 59,879 | 104,241
Corporate securities | Investments due in 12 months or greater: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 27,806 | 37,436
Gross Unrealized Gains | 2 | 0
Gross Unrealized Losses | (22) | (195)
Estimated Fair Market Value | $ 27,786 | 37,241
Government securities | Investments due in 4-12 months: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 9,302
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | (17)
Estimated Fair Market Value |  | $ 9,285

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