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Cash and Investments in Marketable Securities - Portfolio of Cash and Investments in Marketable Securities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Gross Unrealized Gains   $ 1,100
Gross Unrealized Losses   (200)
Cash $ 5,681 6,285
Total cash and investments in marketable securities 955,299 1,198,558
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 891,245  
Gross Unrealized Gains 90  
Gross Unrealized Losses (190)  
Fair Value 891,145 1,003,348
Corporate notes and bonds | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 329,376  
Gross Unrealized Gains 53  
Gross Unrealized Losses (90)  
Fair Value 329,339 687,469
Corporate commercial paper | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 555,989  
Gross Unrealized Gains 35  
Gross Unrealized Losses (100)  
Fair Value 555,924 313,497
Obligations of U.S. government agencies | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Amortized Cost 5,880  
Gross Unrealized Gains 2  
Gross Unrealized Losses 0  
Fair Value 5,882 2,382
Money market funds | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 48,336 179,302
Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 10,137 $ 9,623