FAIR VALUE MEASUREMENTS - Summary of Amortized Costs vs Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Cost	$ 2,271	$ 2,354
Unrealized Gain	149	18
Unrealized Loss	(111)	(101)
Fair Value	2,309	2,271
Japanese Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	2,184	2,267
Unrealized Gain	144	17
Unrealized Loss	(107)	(100)
Fair Value	2,221	2,184
Japanese Government Bonds
Debt Securities, Available-for-sale [Line Items]
Cost	87	87
Unrealized Gain	5	1
Unrealized Loss	(4)	(1)
Fair Value	$ 88	$ 87