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MARKETABLE SECURITIES (Details) Securities in Thousands, $ in Thousands | Dec. 31, 2017 USD ($) Securities | Dec. 31, 2016 USD ($) Securities
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 189,663 | $ 188,543
Gross Unrealized Gains | 0 | 38
Gross Unrealized Losses | (427) | (258)
Estimated Fair Market Value | $ 189,236 | $ 188,323
Weighted Average Interest Rate on Investments | 1.57% | 1.23%
Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 38,485 | $ 46,338
Gross Unrealized Gains | 0 | 2
Gross Unrealized Losses | (16) | (2)
Estimated Fair Market Value | 38,469 | 46,338
Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 113,742 | 78,900
Gross Unrealized Gains | 0 | 15
Gross Unrealized Losses | (216) | (76)
Estimated Fair Market Value | 113,526 | 78,839
Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 37,436 | 63,305
Gross Unrealized Gains | 0 | 21
Gross Unrealized Losses | (195) | (180)
Estimated Fair Market Value | $ 37,241 | $ 63,146
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | Securities | 0 | 0
Commercial paper | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | $ 36,996
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 0
Estimated Fair Market Value |  | 36,996
Commercial paper | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 19,186
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 0
Estimated Fair Market Value |  | 19,186
Corporate securities | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 38,485 | 9,342
Gross Unrealized Gains | 0 | 2
Gross Unrealized Losses | (16) | (2)
Estimated Fair Market Value | 38,469 | 9,342
Corporate securities | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 104,440 | 59,714
Gross Unrealized Gains | 0 | 15
Gross Unrealized Losses | (199) | (76)
Estimated Fair Market Value | 104,241 | 59,653
Corporate securities | Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 37,436 | 63,305
Gross Unrealized Gains | 0 | 21
Gross Unrealized Losses | (195) | (180)
Estimated Fair Market Value | 37,241 | $ 63,146
Government securities | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 9,302 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (17) |
Estimated Fair Market Value | $ 9,285 |

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