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Marketable Securities (Details 2) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Cash equivalents and marketable securities, Continuous unrealized loss position |  |
Less than one year, Fair Value | $ 220,339 | $ 216,628
Unrealized gain (loss), Gross |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (66) | (101)
US Treasury and Government Short-term Debt Securities [Member] |  |
Cash equivalents and marketable securities, Continuous unrealized loss position |  |
Less than one year, Fair Value | 36,490 | 32,630
Unrealized gain (loss), Gross |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (3) | (4)
Corporate Debt Securities [Member] |  |
Cash equivalents and marketable securities, Continuous unrealized loss position |  |
Less than one year, Fair Value | 183,849 | 183,998
Unrealized gain (loss), Gross |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ (63) | $ (97)

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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months. + References No definition available. + Details
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