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Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost $ 520,317 $ 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 2,112,225 1,279,275
Cash and Marketable securities, Gross Unrealized Gains 665 17
Cash and Marketable securities, Gross Unrealized Losses (85) (229)
Cash and Marketable Securities, Total Estimated Fair Value 2,112,805 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 416,099 279,950
Cash, Total Estimated Fair Value 416,099 279,950
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 1,305,145 735,988
Marketable securities, Gross Unrealized Gains 604 12
Marketable securities, Gross Unrealized Losses (49) (175)
Marketable securities, Total Estimated Fair Value 1,305,700 735,825
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 269,278 181,032
Marketable securities, Gross Unrealized Gains 48 4
Marketable securities, Gross Unrealized Losses (32) (36)
Marketable securities, Total Estimated Fair Value 269,294 181,000
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 28,420 35,819
Marketable securities, Gross Unrealized Gains 13 1
Marketable securities, Gross Unrealized Losses (4) (18)
Marketable securities, Total Estimated Fair Value 28,429 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 84,498 33,193
Marketable securities, Total Estimated Fair Value 84,498 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 8,785 13,293
Marketable securities, Total Estimated Fair Value $ 8,785 $ 13,293