v3.19.3
Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost $ 728,828 $ 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 2,260,258 1,279,275
Cash and Marketable securities, Gross Unrealized Gains 592 17
Cash and Marketable securities, Gross Unrealized Losses (120) (229)
Cash and Marketable Securities, Total Estimated Fair Value 2,260,730 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 468,196 279,950
Cash, Total Estimated Fair Value 468,196 279,950
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 1,314,132 735,988
Marketable securities, Gross Unrealized Gains 522 12
Marketable securities, Gross Unrealized Losses (91) (175)
Marketable securities, Total Estimated Fair Value 1,314,563 735,825
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 351,511 181,032
Marketable securities, Gross Unrealized Gains 27 4
Marketable securities, Gross Unrealized Losses (29) (36)
Marketable securities, Total Estimated Fair Value 351,509 181,000
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 31,457 35,819
Marketable securities, Gross Unrealized Gains 41 1
Marketable securities, Gross Unrealized Losses   (18)
Marketable securities, Total Estimated Fair Value 31,498 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 72,726 33,193
Marketable securities, Gross Unrealized Gains 2  
Marketable securities, Total Estimated Fair Value 72,728 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 22,236 13,293
Marketable securities, Total Estimated Fair Value $ 22,236 $ 13,293