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Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost $ 1,251,721 $ 520,317
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 2,827,818 2,112,225
Cash and Marketable securities, Gross Unrealized Gains 1,860 665
Cash and Marketable securities, Gross Unrealized Losses (109) (85)
Cash and Marketable Securities, Total Estimated Fair Value 2,829,569 2,112,805
Fair Value, Measurements, Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost 1,189,011 416,099
Cash, Total Estimated Fair Value 1,189,011 416,099
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 1,263,863 1,305,145
Marketable securities, Gross Unrealized Gains 1,735 604
Marketable securities, Gross Unrealized Losses (46) (49)
Marketable securities, Total Estimated Fair Value 1,265,552 1,305,700
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 339,231 269,278
Marketable securities, Gross Unrealized Gains 111 48
Marketable securities, Gross Unrealized Losses (63) (32)
Marketable securities, Total Estimated Fair Value 339,279 269,294
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 22,407 28,420
Marketable securities, Gross Unrealized Gains 14 13
Marketable securities, Gross Unrealized Losses   (4)
Marketable securities, Total Estimated Fair Value 22,421 28,429
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 8,389 84,498
Marketable securities, Total Estimated Fair Value 8,389 84,498
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 4,917 8,785
Marketable securities, Total Estimated Fair Value $ 4,917 $ 8,785