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
Sep. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities $ 47,574 $ 30,991
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 223,874 173,970
Gross unrealized gains/(losses) (18) (26)
Fair value 223,856 173,944
Cash and cash equivalents 176,282 142,953
Marketable securities 47,574 30,991
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 25,257 46,237
Fair value 25,257 46,237
Cash and cash equivalents 25,257 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 151,025 93,430
Fair value 151,025 93,430
Cash and cash equivalents 151,025 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   11,010
Gross unrealized gains/(losses)   (4)
Fair value   11,006
Cash and cash equivalents   490
Marketable securities   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 16,106 20,789
Gross unrealized gains/(losses) (15) (21)
Fair value 16,091 20,768
Cash and cash equivalents   2,796
Marketable securities 16,091 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 31,486 499
Gross unrealized gains/(losses) (3)  
Fair value 31,483 499
Marketable securities $ 31,483 $ 499