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Financial Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Mar. 31, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]        
Document Period End Date Mar. 31, 2020      
Cash $ 24,850 $ 33,238    
Time deposits 5,044 3,924    
Money Market Funds (level 1) 128,213 149,624    
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value 989,934 1,040,678    
Total 1,148,041 1,227,464    
Included in Cash and cash equivalents 498,693 508,196    
Restricted Cash Equivalents, Noncurrent 9,409 9,405    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents 508,102 517,601 $ 788,980 $ 366,753
Included in Marketable securities $ 639,939 $ 709,863