FAIR VALUE MEASUREMENTS - Amortized cost vs fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cost	$ 2,309	$ 2,271
Unrealized Gain	196	149
Unrealized Loss	(119)	(111)
Fair Value	2,386	2,309
Japanese Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	2,221	2,184
Unrealized Gain	189	144
Unrealized Loss	(114)	(107)
Fair Value	2,296	2,221
Japanese Government Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	88	87
Unrealized Gain	7	5
Unrealized Loss	(5)	(4)
Fair Value	$ 90	$ 88