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Short-term Investments | 9 Months Ended
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Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract] |
Short-term Investments | Short-term Investments The Company's investments are classified as available-for-sale. The following table summarizes the amortized cost, gross unrealized gains and losses, and fair value of available-for-sale investments as of September 30, 2019:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Aggregate Fair Value
U.S. government debt securities | $ | 44,925 |  |  | $ | 20 |  |  | $ | — |  |  | $ | 44,945 |
Certificates of deposit | 340 |  |  | — |  |  | — |  |  | 340 |
Total | $ | 45,265 |  |  | $ | 20 |  |  | $ | — |  |  | $ | 45,285 |
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 |
As of September 30, 2019, all of the available-for-sale investments were due within one year based on their contractual maturities. Changes in market interest rates and bond yields cause certain investments to fall below their cost basis, resulting in unrealized losses on investments. The unrealized losses of the Company's investments were primarily a result of unfavorable changes in interest rates subsequent to the initial purchase of these investments, and there were no unrealized losses as of September 30, 2019. As of September 30, 2019 and December 31, 2018, the Company did not consider any of its debt security investments to be other-than-temporarily impaired. When evaluating its debt security investments for other-than-temporary impairment, the Company reviews factors such as the length of time and extent to which fair value has been below its cost basis, the financial condition of the issuer, the Company's ability and intent to hold the security and whether it is more likely than not that it will be required to sell the investment before recovery of its cost basis.

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- Definition The entire disclosure for investments in certain debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2209399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2324412 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 320 -URI http://asc.fasb.org/subtopic&trid=2176304 + Details
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