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Assets - Fair Value (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Cash equivalents |  |
Total | $ 213,506 | $ 279,736
Short-term investments |  |
Total | 52,063 | 47,780
Transfer of assets from Level 1 to Level 2 | 0 |
Transfer of assets from Level 2 to Level 1 | 0 |
Transfer of Liabilities from Level 1 to Level 2 | 0 |
Transfer of Liabilities from Level 2 to Level 1 | 0 |
Measured at fair value on a recurring basis |  |
Cash equivalents |  |
Total |  | 12,490
Short-term investments |  |
Total | 52,063 | 47,780
Measured at fair value on a recurring basis | U.S. treasuries |  |
Cash equivalents |  |
Total |  | 12,490
Short-term investments |  |
Total | 36,527 | 33,895
Measured at fair value on a recurring basis | Corporate debt |  |
Short-term investments |  |
Total | 8,561 | 10,886
Measured at fair value on a recurring basis | Commercial paper |  |
Short-term investments |  |
Total | 6,975 | 2,999
Measured at fair value on a recurring basis | Level 1 |  |
Cash equivalents |  |
Total |  | 12,490
Short-term investments |  |
Total | 36,527 | 33,895
Measured at fair value on a recurring basis | Level 1 | U.S. treasuries |  |
Cash equivalents |  |
Total |  | 12,490
Short-term investments |  |
Total | 36,527 | 33,895
Measured at fair value on a recurring basis | Level 2 |  |
Short-term investments |  |
Total | 15,536 | 13,885
Measured at fair value on a recurring basis | Level 2 | Corporate debt |  |
Short-term investments |  |
Total | 8,561 | 10,886
Measured at fair value on a recurring basis | Level 2 | Commercial paper |  |
Short-term investments |  |
Total | $ 6,975 | $ 2,999

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