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Financial Instruments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
Dec. 31, 2021
Dec. 31, 2020
Schedule of Available-for-sale Securities [Line Items]    
Total investments in debt securities $ 717,085,000  
Cash, cash equivalents and investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 1,045,488,000 $ 318,729,000
Gross unrealized gains 10,000 106,000
Gross unrealized losses (1,097,000) (21,000)
Total investments in debt securities 1,044,400,000 318,814,000
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 460,731,000 81,612,000
Gross unrealized losses (5,000) (1,000)
Total investments in debt securities 460,725,000 81,611,000
Cash and money market funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 327,316,000 43,040,000
Total investments in debt securities 327,316,000 43,040,000
Commercial paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 133,190,000 32,538,000
Gross unrealized losses (5,000) (1,000)
Total investments in debt securities 133,185,000 32,537,000
U.S. government and agency securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 225,000 170,000
Total investments in debt securities 224,000 170,000
U.S. Treasury security [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost   5,864,000
Total investments in debt securities   5,864,000
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 584,757,000 237,117,000
Gross unrealized gains 10,000 106,000
Gross unrealized losses (1,092,000) (20,000)
Total investments in debt securities 583,675,000 237,203,000
Commercial paper, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 187,705,000 112,648,000
Gross unrealized gains   4,000
Gross unrealized losses (73,000) (8,000)
Total investments in debt securities 187,632,000 112,644,000
Corporate debt securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 8,964,000 17,360,000
Gross unrealized gains 9,000 96,000
Gross unrealized losses (5,000)  
Total investments in debt securities 8,968,000 17,456,000
U.S. government & agency securities, not included with cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 388,088,000 107,109,000
Gross unrealized gains 1,000 6,000
Gross unrealized losses (1,014,000) (12,000)
Total investments in debt securities 387,075,000 107,103,000
Short-Term Restricted Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 500,000 836,000
Total investments in debt securities 500,000 836,000
Long-term restricted cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 4,592,000 3,500,000
Total investments in debt securities $ 4,592,000 $ 3,500,000