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Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Cash, Cost or Amortized Cost $ 245,639 $ 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 1,208,536   $ 1,279,275
Cash and Marketable securities, Gross Unrealized Gains 254   17
Cash and Marketable securities, Gross Unrealized Losses (58)   (229)
Cash and Marketable Securities, Total Estimated Fair Value 1,208,732   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 210,507   279,950
Cash, Total Estimated Fair Value 210,507   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 728,749   735,988
Marketable securities, Gross Unrealized Gains 208   12
Marketable securities, Gross Unrealized Losses (27)   (175)
Marketable securities, Total Estimated Fair Value 728,930   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 167,044   181,032
Marketable securities, Gross Unrealized Gains 15   4
Marketable securities, Gross Unrealized Losses (31)   (36)
Marketable securities, Total Estimated Fair Value 167,028   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 28,931   35,819
Marketable securities, Gross Unrealized Gains 31   1
Marketable securities, Gross Unrealized Losses     (18)
Marketable securities, Total Estimated Fair Value 28,962   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 46,215   33,193
Marketable securities, Total Estimated Fair Value 46,215   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 27,090   13,293
Marketable securities, Total Estimated Fair Value $ 27,090   $ 13,293