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, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 196,634 $ 142,953
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 240,358 173,970
Gross unrealized gains (losses) (9) (26)
Fair value 240,349 173,944
Cash and cash equivalents 196,634 142,953
Marketable securities 43,715 30,991
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 33,671 46,237
Fair value 33,671 46,237
Cash and cash equivalents 33,671 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 158,438 93,430
Fair value 158,438 93,430
Cash and cash equivalents 158,438 93,430
Fair Value Measurements Recurring | Level 2 | Corporate Bonds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 13,629 20,789
Gross unrealized gains (losses) (16) (21)
Fair value 13,613 20,768
Cash and cash equivalents   2,796
Marketable securities 13,613 17,972
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 34,620 499
Gross unrealized gains (losses) 7  
Fair value 34,627 499
Cash and cash equivalents 4,525  
Marketable securities $ 30,102 499
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 | 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