Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis for Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and cash equivalents $ 156,131 $ 196,634
Fair Value Measurements Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 200,620 240,358
Gross unrealized gains/(losses) 25 (9)
Fair value 200,645 240,349
Cash and cash equivalents 156,131 196,634
Marketable securities 44,514 43,715
Fair Value Measurements Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 43,843 33,671
Fair value 43,843 33,671
Cash and cash equivalents 43,843 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 109,288 158,438
Fair value 109,288 158,438
Cash and cash equivalents 109,288 158,438
Fair Value Measurements Recurring | Level 2 | Corporate Bonds    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Adjusted cost 6,530 13,629
Gross unrealized gains/(losses) 7 (16)
Fair value 6,537 13,613
Marketable securities 6,537 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 40,959 34,620
Gross unrealized gains/(losses) 18 7
Fair value 40,977 34,627
Cash and cash equivalents 3,000 4,525
Marketable securities $ 37,977 $ 30,102