5. FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Feb. 28, 2019	May 31, 2018
Assets
Investment securities	$ 10,581	$ 13,796
Level 1
Assets
Investment securities	10,581	13,796
Level 2
Assets
Investment securities	0	0
Level 3
Assets
Investment securities	0	0
Money Market Funds
Assets
Investment securities	10,581	7,813
Money Market Funds | Level 1
Assets
Investment securities	10,581	7,813
Money Market Funds | Level 2
Assets
Investment securities	0	0
Money Market Funds | Level 3
Assets
Investment securities	$ 0	0
U.S. Treasury Securities
Assets
Investment securities		5,983
U.S. Treasury Securities | Level 1
Assets
Investment securities		5,983
U.S. Treasury Securities | Level 2
Assets
Investment securities		0
U.S. Treasury Securities | Level 3
Assets
Investment securities		$ 0