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Short-term Investments | 9 Months Ended
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Sep. 30, 2020
Short Term Investments [Abstract] |
Short-term Investments | 5. Short-term Investments The following is a summary of our short-term investments (in thousands):
|  | September 30, 2020 |
|  |  |  |  |  | Gross |  |  | Gross |  |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost |  |  | Gains |  |  | Losses |  |  | Fair Value |
U.S. treasury bills and government agency obligations |  | $ | 70,175 |  |  | $ | 316 |  |  | $ | — |  |  | $ | 70,491 |
U.S. corporate debt securities |  |  | 25,624 |  |  |  | 110 |  |  |  | — |  |  |  | 25,734 |
Foreign corporate debt securities |  |  | 23,534 |  |  |  | 114 |  |  |  | — |  |  |  | 23,648 |
U.S. commercial paper |  |  | 14,995 |  |  |  | — |  |  |  | — |  |  |  | 14,995 |
Foreign commercial paper |  |  | 28,137 |  |  |  | — |  |  |  | — |  |  |  | 28,137 |
Total |  | $ | 162,465 |  |  | $ | 540 |  |  | $ | — |  |  | $ | 163,005 |
|  | December 31, 2019 |
|  |  |  |  |  | Gross |  |  | Gross |  |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost |  |  | Gains |  |  | Losses |  |  | Fair Value |
U.S. treasury bills and government agency obligations |  | $ | 140,567 |  |  | $ | 59 |  |  | $ | — |  |  | $ | 140,626 |
U.S. corporate debt securities |  |  | 80,159 |  |  |  | 11 |  |  |  | — |  |  |  | 80,170 |
Foreign corporate debt securities |  |  | 23,188 |  |  |  | 15 |  |  |  | — |  |  |  | 23,203 |
U.S. commercial paper |  |  | 32,801 |  |  |  | — |  |  |  | — |  |  |  | 32,801 |
Foreign commercial paper |  |  | 42,274 |  |  |  | — |  |  |  | — |  |  |  | 42,274 |
Total |  | $ | 318,989 |  |  | $ | 85 |  |  | $ | — |  |  | $ | 319,074 |
The amortized cost of debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. We regularly monitor and evaluate the realizable value of our marketable securities. We did not recognize any impairment losses during the three and nine months ended September 30, 2020 and 2019. Realized gains and losses associated with our available-for-sale securities, if any, are reported in the statements of operations and comprehensive loss. We did not recognize any realized gains or losses during the three months ended September 30, 2020 and 2019. We did not recognize any realized gains or losses during the nine months ended September 30, 2020. We recognized $8,000 of realized gains during the nine months ended September 30, 2019.

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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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