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Cash and Cash Equivalents and Investments (Details 1) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Cash and Cash Equivalents |  |
Fair Value |  |
Amortized Cost | $ 48,902 | $ 61,661
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Estimated Fair Value | 48,902 | 61,661
U.S. Treasury Securities |  |
Fair Value |  |
Amortized Cost | 125,651 | 222,316
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (63) | (168)
Estimated Fair Value | 125,588 | 222,148
Corporate Debt Securities |  |
Fair Value |  |
Amortized Cost | 12,815 | 27,033
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (8) | (54)
Estimated Fair Value | 12,807 | 26,979
Total Short-term Investments |  |
Fair Value |  |
Amortized Cost | 138,466 | 249,349
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (71) | (222)
Estimated Fair Value | 138,395 | 249,127
Total Cash and Cash Equivalents and Investments |  |
Fair Value |  |
Amortized Cost | 187,368 | 311,010
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (71) | (222)
Estimated Fair Value | $ 187,297 | $ 310,788

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