11. SEGMENT AND GEOGRAPHIC INFORMATION - Revenue (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Net revenues and long-lived assets by geographic area
Net revenues	$ 12,686	$ 14,702	$ 12,832	$ 11,266	$ 11,191	$ 11,118	$ 9,647	$ 10,687	$ 51,486	$ 42,643	$ 48,180
Number of reporting units | segment									1
United States
Net revenues and long-lived assets by geographic area
Net revenues									$ 19,327	20,690	19,708
China
Net revenues and long-lived assets by geographic area
Net revenues									4,458	5,520	9,364
Singapore
Net revenues and long-lived assets by geographic area
Net revenues									7,592	6,878	9,475
Netherlands
Net revenues and long-lived assets by geographic area
Net revenues									11,093	4,375	4,728
Germany
Net revenues and long-lived assets by geographic area
Net revenues									7,478	3,769	3,226
Rest of the world
Net revenues and long-lived assets by geographic area
Net revenues									$ 1,538	$ 1,411	$ 1,679