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Assets - Fair Value (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Cash equivalents    
Total $ 213,506 $ 279,736
Short-term investments    
Total 52,063 47,780
Transfer of assets from Level 1 to Level 2 0  
Transfer of assets from Level 2 to Level 1 0  
Transfer of Liabilities from Level 1 to Level 2 0  
Transfer of Liabilities from Level 2 to Level 1 0  
Measured at fair value on a recurring basis    
Cash equivalents    
Total   12,490
Short-term investments    
Total 52,063 47,780
Measured at fair value on a recurring basis | U.S. treasuries    
Cash equivalents    
Total   12,490
Short-term investments    
Total 36,527 33,895
Measured at fair value on a recurring basis | Corporate debt    
Short-term investments    
Total 8,561 10,886
Measured at fair value on a recurring basis | Commercial paper    
Short-term investments    
Total 6,975 2,999
Measured at fair value on a recurring basis | Level 1    
Cash equivalents    
Total   12,490
Short-term investments    
Total 36,527 33,895
Measured at fair value on a recurring basis | Level 1 | U.S. treasuries    
Cash equivalents    
Total   12,490
Short-term investments    
Total 36,527 33,895
Measured at fair value on a recurring basis | Level 2    
Short-term investments    
Total 15,536 13,885
Measured at fair value on a recurring basis | Level 2 | Corporate debt    
Short-term investments    
Total 8,561 10,886
Measured at fair value on a recurring basis | Level 2 | Commercial paper    
Short-term investments    
Total $ 6,975 $ 2,999