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Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost $ 967,519 $ 545,618
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 2,556,513 2,537,472
Cash and Marketable securities, Gross Unrealized Gains 202 131
Cash and Marketable securities, Gross Unrealized Losses (18) (63)
Cash and Marketable Securities, Total Estimated Fair Value 2,556,697 2,537,540
Fair Value, Measurements, Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost 930,155 464,006
Cash, Total Estimated Fair Value 930,155 464,006
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 1,002,193 1,272,125
Marketable securities, Gross Unrealized Gains 193 122
Marketable securities, Gross Unrealized Losses   (21)
Marketable securities, Total Estimated Fair Value 1,002,386 1,272,226
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 123,354 245,055
Marketable securities, Gross Unrealized Gains 6 8
Marketable securities, Gross Unrealized Losses (6) (24)
Marketable securities, Total Estimated Fair Value 123,354 245,039
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 68,483 81,158
Marketable securities, Gross Unrealized Gains 3 1
Marketable securities, Gross Unrealized Losses (12) (18)
Marketable securities, Total Estimated Fair Value 68,474 81,141
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 397,233 425,861
Marketable securities, Total Estimated Fair Value 397,233 425,861
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 35,095 49,267
Marketable securities, Total Estimated Fair Value $ 35,095 $ 49,267