v3.19.2
Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost $ 335,744 $ 387,149
Fair Value, Measurements, Recurring    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash and Marketable securities, Cost or Amortized Cost 1,184,189 1,279,275
Cash and Marketable securities, Gross Unrealized Gains 636 17
Cash and Marketable securities, Gross Unrealized Losses (31) (229)
Cash and Marketable Securities, Total Estimated Fair Value 1,184,794 1,279,063
Fair Value, Measurements, Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost 275,644 279,950
Cash, Total Estimated Fair Value 275,644 279,950
Fair Value, Measurements, Recurring | Level 1 Securities | U.S. Government Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, Cost or Amortized Cost 630,566 735,988
Marketable securities, Gross Unrealized Gains 541 12
Marketable securities, Gross Unrealized Losses (12) (175)
Marketable securities, Total Estimated Fair Value 631,095 735,825
Fair Value, Measurements, Recurring | Level 1 Securities | U.S. Government Agency Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, Cost or Amortized Cost 154,109 181,032
Marketable securities, Gross Unrealized Gains 39 4
Marketable securities, Gross Unrealized Losses (19) (36)
Marketable securities, Total Estimated Fair Value 154,129 181,000
Fair Value, Measurements, Recurring | Level 2 Securities | Corporate Debt Securities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, Cost or Amortized Cost 30,713 35,819
Marketable securities, Gross Unrealized Gains 56 1
Marketable securities, Gross Unrealized Losses   (18)
Marketable securities, Total Estimated Fair Value 30,769 35,802
Fair Value, Measurements, Recurring | Level 2 Securities | Commercial Paper    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, Cost or Amortized Cost 55,706 33,193
Marketable securities, Total Estimated Fair Value 55,706 33,193
Fair Value, Measurements, Recurring | Level 2 Securities | Certificates of Deposit    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Marketable securities, Cost or Amortized Cost 37,451 13,293
Marketable securities, Total Estimated Fair Value $ 37,451 $ 13,293