Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details)

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Fair Value Measurements - Fair Value of Financial Assets and Liabilities Measured on Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets:    
Total $ 184,421 $ 195,396
Level 1    
Assets:    
Total 26,721 116,202
Level 2    
Assets:    
Total 157,700 79,194
Level 3    
Assets:    
Total 0 0
Money Market Funds    
Assets:    
Total 26,721 116,202
Money Market Funds | Level 1    
Assets:    
Total 26,721 116,202
Money Market Funds | Level 2    
Assets:    
Total 0 0
Money Market Funds | Level 3    
Assets:    
Total 0 0
Commercial Paper    
Assets:    
Total 55,085 26,625
Commercial Paper | Level 1    
Assets:    
Total 0 0
Commercial Paper | Level 2    
Assets:    
Total 55,085 26,625
Commercial Paper | Level 3    
Assets:    
Total 0 0
Corporate Debt Securities    
Assets:    
Total 38,998 11,774
Corporate Debt Securities | Level 1    
Assets:    
Total 0 0
Corporate Debt Securities | Level 2    
Assets:    
Total 38,998 11,774
Corporate Debt Securities | Level 3    
Assets:    
Total 0 0
Asset-backed Securities    
Assets:    
Total 30,862 16,899
Asset-backed Securities | Level 1    
Assets:    
Total 0 0
Asset-backed Securities | Level 2    
Assets:    
Total 30,862 16,899
Asset-backed Securities | Level 3    
Assets:    
Total 0 0
U.S. Government Agency Securities    
Assets:    
Total 32,755 23,896
U.S. Government Agency Securities | Level 1    
Assets:    
Total 0 0
U.S. Government Agency Securities | Level 2    
Assets:    
Total 32,755 23,896
U.S. Government Agency Securities | Level 3    
Assets:    
Total $ 0 $ 0