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Assets - Fair Value (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Cash equivalents          
Total $ 205,564 $ 205,564 $ 277,444 $ 257,641 $ 269,232
Short-term investments          
Total 97,086 97,086 66,787    
Transfer of assets between levels 0 0      
Assets measured on a recurring basis          
Cash equivalents          
Total     1,501    
Short-term investments          
Total 97,086 97,086 66,787    
Assets measured on a recurring basis | U.S. treasuries          
Short-term investments          
Total 34,951 34,951 40,008    
Assets measured on a recurring basis | Government agency securities          
Short-term investments          
Total 62,135 62,135 10,012    
Assets measured on a recurring basis | Corporate debt          
Cash equivalents          
Total     1,501    
Short-term investments          
Total     13,773    
Assets measured on a recurring basis | Commercial paper          
Short-term investments          
Total     2,994    
Assets measured on a recurring basis | Level 1          
Short-term investments          
Total 34,951 34,951 40,008    
Assets measured on a recurring basis | Level 1 | U.S. treasuries          
Short-term investments          
Total 34,951 34,951 40,008    
Assets measured on a recurring basis | Level 2          
Cash equivalents          
Total     1,501    
Short-term investments          
Total 62,135 62,135 26,779    
Assets measured on a recurring basis | Level 2 | Government agency securities          
Short-term investments          
Total $ 62,135 $ 62,135 10,012    
Assets measured on a recurring basis | Level 2 | Corporate debt          
Cash equivalents          
Total     1,501    
Short-term investments          
Total     13,773    
Assets measured on a recurring basis | Level 2 | Commercial paper          
Short-term investments          
Total     $ 2,994