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Marketable Securities (Details 2) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Cash equivalents and marketable securities, Continuous unrealized loss position |  |
Less than one year, Fair Value | $ 219,699 | $ 236,596
Unrealized gain (loss), Gross |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (184) | (132)
US Treasury and Government Short-term Debt Securities [Member] |  |
Cash equivalents and marketable securities, Continuous unrealized loss position |  |
Less than one year, Fair Value | 22,913 | 42,581
Unrealized gain (loss), Gross |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | (3) | (1)
Corporate Debt Securities [Member] |  |
Cash equivalents and marketable securities, Continuous unrealized loss position |  |
Less than one year, Fair Value | 196,786 | 194,015
Unrealized gain (loss), Gross |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ (181) | $ (131)

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