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Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Fair Value Measurements, Financial Assets    
Marketable securities, Amortized Cost $ 301,899 $ 176,185
Marketable securities, Aggregate Fair Value 301,136 175,587
Recurring    
Fair Value Measurements, Financial Assets    
Marketable securities, Amortized Cost 301,899 176,185
Marketable securities, Gross Unrealized Losses Less Than 12 Months (712) (598)
Marketable securities, Gross Unrealized Losses More Than 12 Months (51)  
Marketable securities, Aggregate Fair Value 301,136 175,587
Total financial assets, Amortized Cost or Carrying Value 765,940 271,617
Total financial assets 765,177 271,019
Recurring | U.S. Treasury securities    
Fair Value Measurements, Financial Assets    
Marketable securities, Amortized Cost 69,898 59,962
Marketable securities, Gross Unrealized Losses Less Than 12 Months (59) (216)
Marketable securities, Gross Unrealized Losses More Than 12 Months (51)  
Marketable securities, Aggregate Fair Value 69,788 59,746
Recurring | Corporate debt Securities and commercial paper    
Fair Value Measurements, Financial Assets    
Marketable securities, Amortized Cost 232,001 116,223
Marketable securities, Gross Unrealized Losses Less Than 12 Months (653) (382)
Marketable securities, Aggregate Fair Value 231,348 115,841
Recurring | Level 1    
Fair Value Measurements, Financial Assets    
Marketable securities, Aggregate Fair Value 69,788 59,746
Total financial assets 503,924 155,178
Recurring | Level 1 | U.S. Treasury securities    
Fair Value Measurements, Financial Assets    
Marketable securities, Aggregate Fair Value 69,788 59,746
Recurring | Level 2    
Fair Value Measurements, Financial Assets    
Marketable securities, Aggregate Fair Value 231,348 115,841
Total financial assets 261,253 115,841
Recurring | Level 2 | Corporate debt Securities and commercial paper    
Fair Value Measurements, Financial Assets    
Marketable securities, Aggregate Fair Value 231,348 115,841
Recurring | Carrying Value    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 464,041 95,432
Recurring | Carrying Value | Money market funds    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 434,136 95,432
Recurring | Carrying Value | Commercial paper    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 29,905  
Recurring | Aggregate Fair Value    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 464,041 95,432
Recurring | Aggregate Fair Value | Money market funds    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 434,136 95,432
Recurring | Aggregate Fair Value | Commercial paper    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 29,905  
Recurring | Aggregate Fair Value | Level 1    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 434,136 95,432
Recurring | Aggregate Fair Value | Level 1 | Money market funds    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 434,136 $ 95,432
Recurring | Aggregate Fair Value | Level 2    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents 29,905  
Recurring | Aggregate Fair Value | Level 2 | Commercial paper    
Fair Value Measurements, Financial Assets    
Cash and cash equivalents $ 29,905