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Financial Instruments - Cash Equivalents, Restricted Cash and Investments, Classified as Available-for-Sale Securities (Details) - USD ($)
$ in Thousands
Jun. 30, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost $ 1,084,564 $ 535,042
Unrealized Gains 2,339 1,595
Unrealized Losses (65) (8)
Cash equivalents 256,773 122,900
Short-term investments 619,271 355,407 [1]
Long-term investments 210,794 58,322 [1]
Total cash equivalents and investments 1,086,838 536,629
Money market funds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 256,773 122,900
Unrealized Gains 0 0
Unrealized Losses 0 0
Fair Value 256,773 122,900
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 79,822 204,144
Unrealized Gains 90 871
Unrealized Losses (2) (4)
Fair Value 79,910 205,011
Corporate bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 290,898 181,340
Unrealized Gains 1,205 557
Unrealized Losses (48) (3)
Fair Value 292,055 181,894
U.S. treasury securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 422,592 25,658
Unrealized Gains 907 167
Unrealized Losses (13) (1)
Fair Value 423,486 25,824
U.S. agency securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost 34,479  
Unrealized Gains 137  
Unrealized Losses (2)  
Fair Value $ 34,614  
Certificates of deposit    
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost   1,000
Unrealized Gains   0
Unrealized Losses   0
Fair Value   $ 1,000
[1] The balance sheet as of December 31, 2019 is derived from the audited financial statements as of that date.