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
Mar. 31, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities $ 34,012 $ 30,991
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 188,340 173,970
Gross unrealized losses (44) (26)
Fair value 188,296 173,944
Cash and cash equivalents 154,284 142,953
Marketable securities 34,012 30,991
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 43,188 46,237
Fair value 43,188 46,237
Cash and cash equivalents 43,188 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 107,906 93,430
Fair value 107,906 93,430
Cash and cash equivalents 107,906 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 11,652 11,010
Gross unrealized losses (1) (4)
Fair value 11,651 11,006
Cash and cash equivalents 3,190 490
Marketable securities 8,461 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 22,086 20,789
Gross unrealized losses (43) (21)
Fair value 22,043 20,768
Cash and cash equivalents   2,796
Marketable securities 22,043 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 1,005 2,005
Gross unrealized losses   (1)
Fair value 1,005 2,004
Marketable securities 1,005 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 2,503 499
Fair value 2,503 499
Marketable securities $ 2,503 $ 499