v3.10.0.1
Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost $ 387,149 $ 334,063
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 1,279,275 2,044,152
Cash and Marketable securities, Gross Unrealized Gains 17 8
Cash and Marketable securities, Gross Unrealized Losses (229) (1,121)
Cash and Marketable Securities, Total Estimated Fair Value 1,279,063 2,043,039
Fair Value, Measurements, Recurring | Cash    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Cash, Cost or Amortized Cost 279,950 267,715
Cash, Total Estimated Fair Value 279,950 267,715
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 735,988 1,377,339
Marketable securities, Gross Unrealized Gains 12 2
Marketable securities, Gross Unrealized Losses (175) (899)
Marketable securities, Total Estimated Fair Value 735,825 1,376,442
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 181,032 399,098
Marketable securities, Gross Unrealized Gains 4 6
Marketable securities, Gross Unrealized Losses (36) (222)
Marketable securities, Total Estimated Fair Value 181,000 398,882
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 35,819 0
Marketable securities, Gross Unrealized Gains 1 0
Marketable securities, Gross Unrealized Losses (18) 0
Marketable securities, Total Estimated Fair Value 35,802 0
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 33,193 0
Marketable securities, Gross Unrealized Gains 0 0
Marketable securities, Gross Unrealized Losses 0 0
Marketable securities, Total Estimated Fair Value 33,193 0
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 13,293 0
Marketable securities, Gross Unrealized Gains 0 0
Marketable securities, Gross Unrealized Losses 0 0
Marketable securities, Total Estimated Fair Value $ 13,293 $ 0