Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Summary of Fair Value Assets Measured on Recurring Basis for Cash, Cash Equivalents and Marketable Securities (Details)

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Summary of Significant Accounting Policies - Summary of Fair Value Assets Measured on Recurring Basis for Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities $ 30,991 $ 21,033
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 173,970 113,914
Gross unrealized losses (26) (30)
Fair value 173,944 113,884
Cash and cash equivalents 142,953 92,851
Marketable securities 30,991 21,033
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 46,237 48,533
Fair value 46,237 48,533
Cash and cash equivalents 46,237 48,533
Fair Value Measurements Recurring | Level 1 | Money Market Accounts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 93,430 39,277
Fair value 93,430 39,277
Cash and cash equivalents 93,430 39,277
Fair Value Measurements Recurring | Level 2 | Certificates Of Deposit    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 11,010 15,904
Gross unrealized losses (4) (8)
Fair value 11,006 15,896
Cash and cash equivalents 490 5,041
Marketable securities 10,516 10,855
Fair Value Measurements Recurring | Level 2 | Corporate Bonds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 20,789 10,200
Gross unrealized losses (21) (22)
Fair value 20,768 10,178
Cash and cash equivalents 2,796  
Marketable securities 17,972 $ 10,178
Fair Value Measurements Recurring | Level 2 | Agency Mortgage-backed Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 2,005  
Gross unrealized losses (1)  
Fair value 2,004  
Marketable securities 2,004  
Fair Value Measurements Recurring | Level 2 | U.S. Treasury Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 499  
Fair value 499  
Marketable securities $ 499