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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