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Assets - Fair Value (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Cash equivalents |  |
Total | $ 245,525 | $ 279,736
Short-term investments |  |
Total | 65,130 | 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 |
Recurring |  |
Cash equivalents |  |
Total | 6,496 | 12,490
Short-term investments |  |
Total | 65,130 | 47,780
Recurring | U.S. treasuries |  |
Cash equivalents |  |
Total | 6,496 | 12,490
Short-term investments |  |
Total | 42,228 | 33,895
Recurring | Corporate debt |  |
Short-term investments |  |
Total | 10,522 | 10,886
Recurring | Commercial paper |  |
Short-term investments |  |
Total | 12,380 | 2,999
Recurring | Level 1 |  |
Cash equivalents |  |
Total | 6,496 | 12,490
Short-term investments |  |
Total | 42,228 | 33,895
Recurring | Level 1 | U.S. treasuries |  |
Cash equivalents |  |
Total | 6,496 | 12,490
Short-term investments |  |
Total | 42,228 | 33,895
Recurring | Level 2 |  |
Short-term investments |  |
Total | 22,902 | 13,885
Recurring | Level 2 | Corporate debt |  |
Short-term investments |  |
Total | 10,522 | 10,886
Recurring | Level 2 | Commercial paper |  |
Short-term investments |  |
Total | $ 12,380 | $ 2,999

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