FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash	$ 3,802	$ 6,467
Adjusted Cost	73,136	78,027
Unrealized Gains	164	247
Fair Value	73,300	78,274
Cash and cash equivalents	32,415	16,596
Short-term Marketable Securities	40,885	61,678
U.S. Treasury Securities, maturities less than three months
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	28,613	10,129
Fair Value	28,613	10,129
Cash equivalents	28,613	10,129
U.S. Treasury Securities, maturities greater than three months
Debt Securities, Available-for-sale [Line Items]
Adjusted Cost	40,721	61,431
Unrealized Gains	164	247
Fair Value	40,885	61,678
Short-term Marketable Securities	$ 40,885	$ 61,678