Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value of Financial Instruments - Summary of Fair Value Assets Measured on Recurring Basis for Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities $ 42,068 $ 30,991
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 208,435 173,970
Gross unrealized gains/(losses) (23) (26)
Fair value 208,412 173,944
Cash and cash equivalents 166,344 142,953
Marketable securities 42,068 30,991
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 45,660 46,237
Fair value 45,660 46,237
Cash and cash equivalents 45,660 46,237
Fair Value Measurements Recurring | Level 1 | Money Market Accounts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 120,684 93,430
Fair value 120,684 93,430
Cash and cash equivalents 120,684 93,430
Fair Value Measurements Recurring | Level 2 | Certificates Of Deposit    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 1,476 11,010
Gross unrealized gains/(losses) (1) (4)
Fair value 1,475 11,006
Cash and cash equivalents   490
Marketable securities 1,475 10,516
Fair Value Measurements Recurring | Level 2 | Corporate Bonds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 18,141 20,789
Gross unrealized gains/(losses) (27) (21)
Fair value 18,114 20,768
Cash and cash equivalents   2,796
Marketable securities 18,114 17,972
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 gains/(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 22,474 499
Gross unrealized gains/(losses) 5  
Fair value 22,479 499
Marketable securities $ 22,479 $ 499