Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurements - Summary of Financial Assets and Liabilities Measured and Recognized at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost $ 16,414  
Fair Value 16,412  
Fair Value, Measurements, Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 118,386 $ 51,046
Unrealized Gains 1  
Unrealized Losses (2)  
Fair Value 118,385 51,046
Cash and cash equivalents 101,973 51,033
Marketable securities 16,412  
Restricted cash   13
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 111,704 51,046
Unrealized Losses (2)  
Fair Value 111,702 51,046
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 5,497 804
Fair Value 5,497 804
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Money Market Funds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 89,793 50,229
Fair Value 89,793 50,229
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | U.S. Treasury Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 16,414  
Unrealized Losses (2)  
Fair Value 16,412  
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Restricted Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost   13
Fair Value   $ 13
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 6,682  
Unrealized Gains 1  
Fair Value 6,683  
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Amortized Cost 6,682  
Unrealized Gains 1  
Fair Value $ 6,683