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Short-term Investments (Tables) | 3 Months Ended
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Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract] |
Summary of Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Short-term Investments | The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of March 31, 2019:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Aggregate Fair Value
U.S. government debt securities | $ | 74,752 |  |  | $ | — |  |  | $ | (14 | ) |  | $ | 74,738 |
Certificates of deposit | 352 |  |  | — |  |  | — |  |  | 352 |
Total | $ | 75,104 |  |  | $ | — |  |  | $ | (14 | ) |  | $ | 75,090 |
The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of December 31, 2018:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Aggregate Fair Value
U.S. government debt securities | $ | 119,401 |  |  | $ | — |  |  | $ | (61 | ) |  | $ | 119,340 |
Certificates of deposit | 348 |  |  | — |  |  | — |  |  | 348 |
Total | $ | 119,749 |  |  | $ | — |  |  | $ | (61 | ) |  | $ | 119,688 |

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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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