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Short-Term Investments | 6 Months Ended
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Jun. 30, 2019
Short Term Investments [Abstract] |
Short-Term Investments | 5. 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 |
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 six months ended June 30, 2019 and 2018. Realized gains and losses associated with our investments, 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 June 30, 2019 and 2018. We recognized $8,000 in realized gains during the six months ended June 30, 2019. We did not recognize any realized gains or losses during the six months ended June 30, 2018.

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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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