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Investment in marketable securities (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]    
Amortized Cost $ 110,277 $ 110,305
Unrealized gains 175 76
Unrealized losses (119) (17)
Fair Value 110,333 110,364
Amortized Cost    
Due within one year 96,225  
Due after one year through two years 14,052  
Total 110,277  
Fair Value    
Due within one year 96,337  
Due after one year through two years 13,996  
Total 110,333  
U.S. treasury bonds    
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]    
Amortized Cost 75,509 75,340
Unrealized gains 175 76
Fair Value 75,684 75,416
Corporate bonds    
Debt Securities, Held-to-maturity, Allowance for Credit Loss [Line Items]    
Amortized Cost 34,768 34,965
Unrealized losses (119) (17)
Fair Value $ 34,649 $ 34,948