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Cash, Cash Equivalents and Investments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 1,160,245 $ 318,729
Gross unrealized gains 94 106
Gross unrealized losses (20) (21)
Fair value 1,160,319 318,814
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 932,400 81,612
Gross unrealized gains
Gross unrealized losses (2) (1)
Fair value 932,398 81,611
Cash and money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 906,453 43,040
Gross unrealized gains
Gross unrealized losses
Fair value 906,453 43,040
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 25,947 32,538
Gross unrealized gains
Gross unrealized losses (2) (1)
Fair value 25,945 32,537
U.S. government and agency securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 136,883 170
Gross unrealized gains 29
Gross unrealized losses (4)
Fair value 136,908 170
U.S. Treasury security [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   5,864
Gross unrealized gains  
Gross unrealized losses  
Fair value   5,864
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 227,845 237,117
Gross unrealized gains 94 106
Gross unrealized losses (18) (20)
Fair value 227,921 237,203
Commercial paper, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 75,470 112,648
Gross unrealized gains 2 4
Gross unrealized losses (7) (8)
Fair value 75,465 112,644
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 15,492 17,360
Gross unrealized gains 63 96
Gross unrealized losses (7)
Fair value 15,548 17,456
U.S. government & agency securities, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   107,109
Gross unrealized gains   6
Gross unrealized losses   (12)
Fair value   107,103
Short-Term Restricted Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 836 836
Gross unrealized gains
Gross unrealized losses
Fair value 836 836
Long-term restricted cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 3,500 3,500
Gross unrealized gains
Gross unrealized losses
Fair value $ 3,500 $ 3,500