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Fair Value Measurements - Fair Value Hierarchy (Details) - Fair Value, Measurements, Recurring - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Assets    
Total assets $ 112,823 $ 99,876
Liabilities    
Total liabilities 0 0
Fair Value, Inputs, Level 1    
Assets    
Total assets 1,156 2,370
Fair Value, Inputs, Level 2    
Assets    
Total assets 111,667 97,506
Money market funds    
Assets    
Total assets 1,156 2,370
Money market funds | Fair Value, Inputs, Level 1    
Assets    
Total assets 1,156 2,370
U.S. Government agency bonds    
Assets    
Total assets 1,982 2,681
U.S. Government agency bonds | Fair Value, Inputs, Level 2    
Assets    
Total assets 1,982 2,681
U.S. Government bonds    
Assets    
Total assets 1,297 1,782
U.S. Government bonds | Fair Value, Inputs, Level 2    
Assets    
Total assets 1,297 1,782
Bank certificates of deposit    
Assets    
Total assets 15,201 10,298
Bank certificates of deposit | Fair Value, Inputs, Level 2    
Assets    
Total assets 15,201 10,298
Commercial paper.    
Assets    
Cash equivalents   3,000
Total assets 9,593 14,593
Commercial paper. | Fair Value, Inputs, Level 2    
Assets    
Total assets 9,593 14,593
Corporate notes    
Assets    
Cash equivalents 1,000  
Total assets 61,752 51,417
Corporate notes | Fair Value, Inputs, Level 2    
Assets    
Total assets 61,752 51,417
Asset-backed securities    
Assets    
Total assets 21,842 16,735
Asset-backed securities | Fair Value, Inputs, Level 2    
Assets    
Total assets $ 21,842 $ 16,735