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MARKETABLE SECURITIES (Schedule of Company's Marketable Securities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Level 2 securities:      
Marketable securities $ 21,652 $ 40,966 $ 56,662
U.S government and agency bonds [Member]      
Level 2 securities:      
Marketable securities 4,192 2,499  
Canada government bonds [Member]      
Level 2 securities:      
Marketable securities 999  
Other foreign government bonds [Member]      
Level 2 securities:      
Marketable securities 2,006 3,521  
Corporate bonds [Member]      
Level 2 securities:      
Marketable securities [1] $ 15,454 $ 33,947  
[1] Investments in Corporate bonds rated A or higher.