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Cash and Cash Equivalents and Investments (Tables) | 9 Months Ended
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Sep. 30, 2018
Cash and Cash Equivalents and Investments [Abstract] |
Schedule of Cash, Cash Equivalents and Short-term Investments [Table Text Block] |
|  | As of September 30, 2018
|  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
|  | (in thousands)
Cash and cash equivalents |  | $ | 48,902 |  |  | $ | — |  |  | $ | — |  |  | $ | 48,902 |
Securities maturing within one year: |  |  |  |  |  |  |  |
U.S. treasury securities |  | 125,651 |  |  | — |  |  | (63 | ) |  | 125,588 |
Corporate debt securities |  | 12,815 |  |  | — |  |  | (8 | ) |  | 12,807 |
Total short-term investments |  | $ | 138,466 |  |  | $ | — |  |  | $ | (71 | ) |  | $ | 138,395 |
Total cash and cash equivalents and investments |  | $ | 187,368 |  |  | $ | — |  |  | $ | (71 | ) |  | $ | 187,297 |
|  | As of December 31, 2017
|  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Estimated Fair Value
|  |  |  | (in thousands) |  |
Cash and cash equivalents |  | $ | 61,661 |  |  | $ | — |  |  | $ | — |  |  | $ | 61,661 |
Securities maturing within one year: |  |  |  |  |  |  |  |
U.S. treasury securities |  | 222,316 |  |  | — |  |  | (168 | ) |  | 222,148 |
Corporate debt securities |  | 27,033 |  |  | — |  |  | (54 | ) |  | 26,979 |
Total short-term investments |  | $ | 249,349 |  |  | $ | — |  |  | $ | (222 | ) |  | $ | 249,127 |
Total cash and cash equivalents and investments |  | $ | 311,010 |  |  | $ | — |  |  | $ | (222 | ) |  | $ | 310,788 |

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- Definition Cash and Cash Equivalents and Investments [Abstract] + References No definition available. + Details
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- Definition Tabular disclosure of the components of cash, cash equivalents, and investments. + References No definition available. + Details
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