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Short-Term Investments (Tables) | 6 Months Ended
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Jun. 30, 2019
Short Term Investments [Abstract] |
Summary of Short-Term Investments | The following is a summary of our short-term investments (in thousands):
|  | June 30, 2019 |
|  |  |  |  |  | Gross |  |  | Gross |  |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost |  |  | Gains |  |  | Losses |  |  | Fair Value |
U.S. treasury bills and government agency obligations |  | $ | 99,524 |  |  | $ | 91 |  |  | $ | — |  |  | $ | 99,615 |
U.S. corporate debt securities |  |  | 33,007 |  |  |  | 30 |  |  |  | — |  |  |  | 33,037 |
Foreign corporate debt securities |  |  | 10,002 |  |  |  | 16 |  |  |  | — |  |  |  | 10,018 |
U.S. commercial paper |  |  | 48,969 |  |  |  | 10 |  |  |  | — |  |  |  | 48,979 |
Foreign commercial paper |  |  | 31,318 |  |  |  | — |  |  |  | — |  |  |  | 31,318 |
Total |  | $ | 222,820 |  |  | $ | 147 |  |  | $ | — |  |  | $ | 222,967 |
|  | December 31, 2018 |
|  |  |  |  |  | Gross |  |  | Gross |  |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost |  |  | Gains |  |  | Losses |  |  | Fair Value |
U.S. treasury bills and government agency obligations |  | $ | 59,747 |  |  | $ | — |  |  | $ | (6 | ) |  | $ | 59,741 |
U.S. corporate debt securities |  |  | 59,164 |  |  |  | — |  |  |  | (77 | ) |  |  | 59,087 |
Foreign corporate debt securities |  |  | 5,041 |  |  |  | 5 |  |  |  | — |  |  |  | 5,046 |
U.S. commercial paper |  |  | 52,800 |  |  |  | — |  |  |  | — |  |  |  | 52,800 |
Foreign commercial paper |  |  | 123,870 |  |  |  | — |  |  |  | (9 | ) |  |  | 123,861 |
Total |  | $ | 300,622 |  |  | $ | 5 |  |  | $ | (92 | ) |  | $ | 300,535 |

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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