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FINANCIAL INSTRUMENTS (Schedule of Classification of Financial Instruments by Fair Value Hierarchy) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Financial assets:    
Marketable securities $ 59,940 $ 71,738
Level 2 [member]    
Financial assets:    
Marketable securities $ 59,940 $ 71,738