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FINANCIAL INSTRUMENTS (Details) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Cash $ 3,500 $ 6,337
Adjusted Cost   77,897
Unrealized Gains   247
Fair Value   78,144
Cash and cash equivalents 67,360 16,466
Short-term Marketable Securities 0 61,678
U.S. Treasury Securities, maturities less than three months    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost $ 63,900 10,129
Fair Value   10,129
Cash equivalents   10,129
U.S. Treasury Securities, maturities greater than three months    
Debt Securities, Available-for-sale [Line Items]    
Adjusted Cost   61,431
Unrealized Gains   247
Fair Value   61,678
Short-term Marketable Securities   $ 61,678