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Fair Value Measurements (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Cash equivalents |  |  |  |
Total | $ 279,736 | $ 277,444 | $ 269,232 | $ 270,811
Assets measured on a recurring basis |  |  |  |
Cash equivalents |  |  |  |
Total | 12,490 | 1,501 |  |
Short-term investments |  |  |  |
Total | 47,780 | 66,787 |  |
Assets measured on a recurring basis | U.S. treasuries |  |  |  |
Short-term investments |  |  |  |
Total | 33,895 | 40,008 |  |
Assets measured on a recurring basis | Government agency securities |  |  |  |
Short-term investments |  |  |  |
Total |  | 10,012 |  |
Assets measured on a recurring basis | Corporate debt |  |  |  |
Cash equivalents |  |  |  |
Total | 12,490 | 1,501 |  |
Short-term investments |  |  |  |
Total | 10,886 | 13,773 |  |
Assets measured on a recurring basis | Commercial paper |  |  |  |
Short-term investments |  |  |  |
Total | 2,999 | 2,994 |  |
Assets measured on a recurring basis | Level 1 |  |  |  |
Cash equivalents |  |  |  |
Total | 12,490 |  |  |
Short-term investments |  |  |  |
Total | 33,895 | 40,008 |  |
Assets measured on a recurring basis | Level 1 | U.S. treasuries |  |  |  |
Short-term investments |  |  |  |
Total | 33,895 | 40,008 |  |
Assets measured on a recurring basis | Level 1 | Corporate debt |  |  |  |
Cash equivalents |  |  |  |
Total | 12,490 |  |  |
Assets measured on a recurring basis | Level 2 |  |  |  |
Cash equivalents |  |  |  |
Total |  | 1,501 |  |
Short-term investments |  |  |  |
Total | 13,885 | 26,779 |  |
Assets measured on a recurring basis | Level 2 | Government agency securities |  |  |  |
Short-term investments |  |  |  |
Total |  | 10,012 |  |
Assets measured on a recurring basis | Level 2 | Corporate debt |  |  |  |
Cash equivalents |  |  |  |
Total |  | 1,501 |  |
Short-term investments |  |  |  |
Total | 10,886 | 13,773 |  |
Assets measured on a recurring basis | Level 2 | Commercial paper |  |  |  |
Short-term investments |  |  |  |
Total | $ 2,999 | $ 2,994 |  |

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