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Financial Instruments (Summary of Cash, Cash Equivalents and Investments) (Details) - USD ($)
Dec. 31, 2022
Dec. 31, 2021
Schedule of Available-for-sale Securities [Line Items]    
Total investments in debt securities $ 634,682,000  
Cash, cash equivalents and investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 777,083,000 $ 1,045,488,000
Gross unrealized gains 16,000 10,000
Gross unrealized losses (4,781,000) (1,097,000)
Total investments in debt securities 772,318,000 1,044,400,000
Cash and cash equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 325,144,000 460,731,000
Gross unrealized gains 6,000  
Gross unrealized losses (61,000) (5,000)
Total investments in debt securities 325,089,000 460,725,000
Cash And Money Market Funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 137,636,000 327,316,000
Total investments in debt securities 137,636,000 327,316,000
Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 166,514,000 133,190,000
Gross unrealized losses (61,000) (5,000)
Total investments in debt securities 166,453,000 133,185,000
U.S. Government And Agency Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 20,994,000 225,000
Gross unrealized gains 6,000  
Total investments in debt securities 21,000,000 224,000
Investments [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 451,939,000 584,757,000
Gross unrealized gains 10,000 10,000
Gross unrealized losses (4,720,000) (1,092,000)
Total investments in debt securities 447,229,000 583,675,000
Commercial Paper, Not Included With Cash And Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 127,626,000 187,705,000
Gross unrealized gains 9,000  
Gross unrealized losses (333,000) (73,000)
Total investments in debt securities 127,302,000 187,632,000
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 49,998,000 8,964,000
Gross unrealized gains   9,000
Gross unrealized losses (507,000) (5,000)
Total investments in debt securities 49,491,000 8,968,000
U.S. Government & Agency Securities, Not Included With Cash And Cash Equivalents [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 274,315,000 388,088,000
Gross unrealized gains 1,000 1,000
Gross unrealized losses (3,880,000) (1,014,000)
Total investments in debt securities 270,436,000 387,075,000
Long-Term Restricted Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 2,922,000 4,592,000
Total investments in debt securities 2,922,000 4,592,000
Short-Term Restricted Cash [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost 300,000 500,000
Total investments in debt securities $ 300,000 $ 500,000