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Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Assets    
Total assets $ 90,863 $ 89,348
Fair Value, Inputs, Level 1    
Assets    
Total assets 443 80
Fair Value, Inputs, Level 2    
Assets    
Total assets 90,420 89,268
Money market funds    
Assets    
Total assets 443 80
Money market funds | Fair Value, Inputs, Level 1    
Assets    
Total assets 443 80
U.S. Government agency bonds    
Assets    
Total assets 3,686 9,515
U.S. Government agency bonds | Fair Value, Inputs, Level 2    
Assets    
Total assets 3,686 9,515
U.S. Government bonds    
Assets    
Total assets 1,787  
U.S. Government bonds | Fair Value, Inputs, Level 2    
Assets    
Total assets 1,787  
Bank certificates of deposit    
Assets    
Total assets 15,458 11,113
Bank certificates of deposit | Fair Value, Inputs, Level 2    
Assets    
Total assets 15,458 11,113
Commercial paper.    
Assets    
Total assets 8,328 17,010
Commercial paper. | Fair Value, Inputs, Level 2    
Assets    
Total assets 8,328 17,010
Corporate notes    
Assets    
Total assets 53,416 45,124
Corporate notes | Fair Value, Inputs, Level 2    
Assets    
Total assets 53,416 45,124
Asset-backed securities    
Assets    
Total assets 7,745 6,506
Asset-backed securities | Fair Value, Inputs, Level 2    
Assets    
Total assets $ 7,745 $ 6,506