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MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 213,117 | $ 188,543
Gross Unrealized Gain | 47 | 38
Gross Unrealized Losses | (122) | (258)
Estimated Fair Market Value | 213,042 | 188,323
Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 28,753 | 46,338
Gross Unrealized Gain | 1 | 2
Gross Unrealized Losses | (2) | (2)
Estimated Fair Market Value | 28,752 | 46,338
Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 148,140 | 78,900
Gross Unrealized Gain | 28 | 15
Gross Unrealized Losses | (83) | (76)
Estimated Fair Market Value | 148,085 | 78,839
Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 36,224 | 63,305
Gross Unrealized Gain | 18 | 21
Gross Unrealized Losses | (37) | (180)
Estimated Fair Market Value | 36,205 | 63,146
Corporate securities | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 19,278 | 9,342
Gross Unrealized Gain | 1 | 2
Gross Unrealized Losses | (2) | (2)
Estimated Fair Market Value | 19,277 | 9,342
Corporate securities | Investments due in 4-12 months: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 138,803 | 59,714
Gross Unrealized Gain | 28 | 15
Gross Unrealized Losses | (78) | (76)
Estimated Fair Market Value | 138,753 | 59,653
Corporate securities | Investments due in 12 months or greater: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 36,224 | 63,305
Gross Unrealized Gain | 18 | 21
Gross Unrealized Losses | (37) | (180)
Estimated Fair Market Value | 36,205 | 63,146
Commercial paper | Investments due in 3 months or less: |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 9,475 | 36,996
Gross Unrealized Gain | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Market Value | 9,475 | 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 | 9,337 |
Gross Unrealized Gain | 0 |
Gross Unrealized Losses | (5) |
Estimated Fair Market Value | $ 9,332 |

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