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, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 213,947 $ 196,634
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 256,101 240,358
Gross unrealized gains/(losses) 48 (9)
Fair value 256,149 240,349
Cash and cash equivalents 213,947 196,634
Marketable securities 42,202 43,715
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 34,834 33,671
Fair value 34,834 33,671
Cash and cash equivalents 34,834 33,671
Fair Value Measurements Recurring | Level 1 | Money Market Accounts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 171,094 158,438
Fair value 171,094 158,438
Cash and cash equivalents 171,094 158,438
Fair Value Measurements Recurring | Level 2 | Corporate Bonds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 10,028 13,629
Gross unrealized gains/(losses) 8 (16)
Fair value 10,036 13,613
Cash and cash equivalents 3,522  
Marketable securities 6,514 13,613
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 40,145 34,620
Gross unrealized gains/(losses) 40 7
Fair value 40,185 34,627
Cash and cash equivalents 4,497 4,525
Marketable securities $ 35,688 $ 30,102