SIGNIFICANT ACCOUNTING POLICIES (Marketable Securities by Contractual Maturity) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Accounting Policies [Abstract]
Marketable securities, due in one year or less, amortized cost	$ 18,684
Marketable securities, due in one year or less, fair value	18,670
Marketable securities, after one year through three years, amortized cost	21,614
Marketable securities, after one year through three years, fair value	21,557
Amortized Cost	40,298
Fair Value	$ 40,227