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MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands | Jun. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 221,468 | $ 188,543
Gross Unrealized Gain | 29 | 38
Gross Unrealized Losses | (151) | (258)
Estimated Fair Market Value | 221,346 | 188,323
Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 42,542 | 46,338
Gross Unrealized Gain | 0 | 2
Gross Unrealized Losses | (5) | (2)
Estimated Fair Market Value | 42,537 | 46,338
Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 110,313 | 78,900
Gross Unrealized Gain | 10 | 15
Gross Unrealized Losses | (88) | (76)
Estimated Fair Market Value | 110,235 | 78,839
Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 68,613 | 63,305
Gross Unrealized Gain | 19 | 21
Gross Unrealized Losses | (58) | (180)
Estimated Fair Market Value | 68,574 | 63,146
Corporate securities | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 24,463 | 9,342
Gross Unrealized Gain | 0 | 2
Gross Unrealized Losses | (5) | (2)
Estimated Fair Market Value | 24,458 | 9,342
Corporate securities | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 110,313 | 59,714
Gross Unrealized Gain | 10 | 15
Gross Unrealized Losses | (88) | (76)
Estimated Fair Market Value | 110,235 | 59,653
Corporate securities | Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 59,241 | 63,305
Gross Unrealized Gain | 19 | 21
Gross Unrealized Losses | (52) | (180)
Estimated Fair Market Value | 59,208 | 63,146
Commercial paper | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 18,079 | 36,996
Gross Unrealized Gain | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 18,079 | 36,996
Commercial paper | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost |  | 19,186
Gross Unrealized Gain |  | 0
Gross Unrealized Losses |  | 0
Estimated Fair Market Value |  | $ 19,186
Government securities | Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 9,372 |
Gross Unrealized Gain | 0 |
Gross Unrealized Losses | (6) |
Estimated Fair Market Value | $ 9,366 |

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