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Assets - Fair Value (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Cash equivalents          
Total $ 235,268 $ 235,268 $ 277,444 $ 274,018 $ 269,232
Short-term investments          
Total 85,853 85,853 66,787    
Transfer of assets between levels 0 0      
Assets measured on a recurring basis          
Cash equivalents          
Total 1,500 1,500 1,501    
Short-term investments          
Total 85,853 85,853 66,787    
Assets measured on a recurring basis | U.S. treasuries          
Short-term investments          
Total 58,806 58,806 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 1,500 1,500 1,501    
Short-term investments          
Total 10,911 10,911 13,773    
Assets measured on a recurring basis | Commercial paper          
Short-term investments          
Total 16,136 16,136 2,994    
Assets measured on a recurring basis | Level 1          
Short-term investments          
Total 58,806 58,806 40,008    
Assets measured on a recurring basis | Level 1 | U.S. treasuries          
Short-term investments          
Total 58,806 58,806 40,008    
Assets measured on a recurring basis | Level 2          
Cash equivalents          
Total 1,500 1,500 1,501    
Short-term investments          
Total 27,047 27,047 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 1,500 1,500 1,501    
Short-term investments          
Total 10,911 10,911 13,773    
Assets measured on a recurring basis | Level 2 | Commercial paper          
Short-term investments          
Total $ 16,136 $ 16,136 $ 2,994