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Fair Value Measurements (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | --- | --- | ---
Cash equivalents |  |  |  |
Total | $ 277,444 | $ 269,232 | $ 270,811 | $ 210,799
Assets measured on a recurring basis |  |  |  |
Cash equivalents |  |  |  |
Total | 1,501 | 17,996 |  |
Short-term investments |  |  |  |
Total | 66,787 | 116,050 |  |
Assets measured on a recurring basis | U.S. treasuries |  |  |  |
Cash equivalents |  |  |  |
Total |  | 9,999 |  |
Short-term investments |  |  |  |
Total | 40,008 | 94,918 |  |
Assets measured on a recurring basis | Government agency securities |  |  |  |
Cash equivalents |  |  |  |
Total |  | 4,998 |  |
Short-term investments |  |  |  |
Total | 10,012 | 12,988 |  |
Assets measured on a recurring basis | Corporate debt |  |  |  |
Cash equivalents |  |  |  |
Total | 1,501 |  |  |
Short-term investments |  |  |  |
Total | 13,773 | 8,144 |  |
Assets measured on a recurring basis | Commercial paper |  |  |  |
Cash equivalents |  |  |  |
Total |  | 2,999 |  |
Short-term investments |  |  |  |
Total | 2,994 |  |  |
Assets measured on a recurring basis | Level 1 |  |  |  |
Cash equivalents |  |  |  |
Total |  | 9,999 |  |
Short-term investments |  |  |  |
Total | 40,008 | 94,918 |  |
Assets measured on a recurring basis | Level 1 | U.S. treasuries |  |  |  |
Cash equivalents |  |  |  |
Total |  | 9,999 |  |
Short-term investments |  |  |  |
Total | 40,008 | 94,918 |  |
Assets measured on a recurring basis | Level 2 |  |  |  |
Cash equivalents |  |  |  |
Total | 1,501 | 7,997 |  |
Short-term investments |  |  |  |
Total | 26,779 | 21,132 |  |
Assets measured on a recurring basis | Level 2 | Government agency securities |  |  |  |
Cash equivalents |  |  |  |
Total |  | 4,998 |  |
Short-term investments |  |  |  |
Total | 10,012 | 12,988 |  |
Assets measured on a recurring basis | Level 2 | Corporate debt |  |  |  |
Cash equivalents |  |  |  |
Total | 1,501 |  |  |
Short-term investments |  |  |  |
Total | 13,773 | 8,144 |  |
Assets measured on a recurring basis | Level 2 | Commercial paper |  |  |  |
Cash equivalents |  |  |  |
Total |  | $ 2,999 |  |
Short-term investments |  |  |  |
Total | $ 2,994 |  |  |

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