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MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 203,889 | $ 188,543
Gross Unrealized Gains | 43 | 38
Gross Unrealized Losses | (179) | (258)
Estimated Fair Market Value | 203,753 | 188,323
Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 31,802 | 46,338
Gross Unrealized Gains | 0 | 2
Gross Unrealized Losses | (14) | (2)
Estimated Fair Market Value | 31,788 | 46,338
Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 85,328 | 78,900
Gross Unrealized Gains | 30 | 15
Gross Unrealized Losses | (27) | (76)
Estimated Fair Market Value | 85,331 | 78,839
Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 86,759 | 63,305
Gross Unrealized Gains | 13 | 21
Gross Unrealized Losses | (138) | (180)
Estimated Fair Market Value | 86,634 | 63,146
Corporate securities | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 21,123 | 9,342
Gross Unrealized Gains | 0 | 2
Gross Unrealized Losses | (14) | (2)
Estimated Fair Market Value | 21,109 | 9,342
Corporate securities | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 67,313 | 59,714
Gross Unrealized Gains | 30 | 15
Gross Unrealized Losses | (27) | (76)
Estimated Fair Market Value | 67,316 | 59,653
Corporate securities | Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 77,353 | 63,305
Gross Unrealized Gains | 13 | 21
Gross Unrealized Losses | (127) | (180)
Estimated Fair Market Value | 77,239 | 63,146
Commercial paper | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 10,679 | 36,996
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 10,679 | 36,996
Commercial paper | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 18,015 | 19,186
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 18,015 | $ 19,186
Government securities | Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 9,406 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (11) |
Estimated Fair Market Value | $ 9,395 |

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