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Cash, Cash Equivalents and Marketable Securities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Cash, Cash Equivalents and Marketable Securities    
Total cash, cash equivalents and marketable securities, Unrealized gains $ 259 $ 14
Total cash, cash equivalents and marketable securities, Unrealized losses (43) (333)
Carrying value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 46,697 57,379
Total cash, cash equivalents and marketable securities, Cost 438,829 484,037
Fair value    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 46,697 57,379
Total cash, cash equivalents and marketable securities, Fair value $ 439,045 $ 483,718
Corporate debt securities due within 3 months of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 3 months 3 months
Corporate debt securities due within 3 months of date of purchase | Carrying value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents   $ 11,974
Corporate debt securities due within 3 months of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents   $ 11,974
Corporate debt securities due within 1 year of date of purchase    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 1 year 1 year
Corporate debt securities due within 1 year of date of purchase | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, unrealized gains $ 220 $ 14
Marketable securities, unrealized losses (40) (333)
Corporate debt securities due within 1 year of date of purchase | Carrying value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, cost 309,365 426,658
Corporate debt securities due within 1 year of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, fair value $ 309,545 426,339
Corporate debt securities due within 1 to 2 years of date of purchase | Maximum [Member]    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 2 years  
Corporate debt securities due within 1 to 2 years of date of purchase | Minimum [Member]    
Cash, Cash Equivalents and Marketable Securities    
Maturity period from date of purchase to classify an investment as marketable securities 1 year  
Corporate debt securities due within 1 to 2 years of date of purchase | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, unrealized gains $ 39  
Marketable securities, unrealized losses (3)  
Corporate debt securities due within 1 to 2 years of date of purchase | Carrying value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, cost 82,767  
Corporate debt securities due within 1 to 2 years of date of purchase | Fair value | Level 2    
Cash, Cash Equivalents and Marketable Securities    
Marketable securities, fair value 82,803  
Cash | Carrying value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 1,772 2,004
Cash | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 1,772 2,004
Money market funds | Carrying value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents 44,925 43,401
Money market funds | Fair value | Level 1    
Cash, Cash Equivalents and Marketable Securities    
Cash and cash equivalents $ 44,925 $ 43,401