6. FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Nov. 30, 2018	May 31, 2018
Money market funds	$ 6,050	$ 7,733
U.S. Treasury securities	5,991	5,983
Certificate of deposit	80	80
Assets	12,121	13,796
Level 1
Money market funds	6,050	7,733
U.S. Treasury securities	5,991	5,983
Certificate of deposit	0	0
Assets	12,041	13,716
Level 2
Money market funds	0	0
U.S. Treasury securities	0	0
Certificate of deposit	80	80
Assets	80	80
Level 3
Money market funds	0	0
U.S. Treasury securities	0	0
Certificate of deposit	0	0
Assets	$ 0	$ 0