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MARKETABLE SECURITIES (Summary of Changes in Fair Value of Company's Marketable Securities) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]      
Balance at beginning of the year $ 40,966 $ 56,662  
Additions 32,322 38,702 $ 71,783
Sale or maturity (51,498) (54,333) (15,092)
Changes in fair value during the year (138) (65) (29)
Balance at end of the period $ 21,652 $ 40,966 $ 56,662