GEOGRAPHIC AND SEGMENT INFORMATION - Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Sales	$ 8,382	$ 7,575	$ 6,649	$ 5,629	$ 5,845	$ 3,672	$ 3,691	$ 3,397	$ 12,278	$ 19,853	$ 10,760	$ 28,235	$ 16,605	$ 10,234
Income (loss) from operations				(5,730)		(2,788)					(8,441)	(20,869)	(14,276)	(13,885)
Interest income				423								1,392	616	0
Interest expense												(8)	(104)	(136)
Other losses, net				(295)		(868)			$ (495)	$ (265)	(868)	(351)	(947)	0
Change in fair value of derivative liabilities											20	0	20	183
Income tax benefit				$ (6)		$ 2,408					$ 2,384	130	3,907	156
Operating Segments | Advanced Energy
Segment Reporting Information [Line Items]
Sales												22,676	12,987	7,636
Income (loss) from operations												(8,045)	(6,326)	(3,957)
Interest income												0	0
Interest expense												0	0	0
Other losses, net												0	0
Change in fair value of derivative liabilities													0	0
Income tax benefit												0	0	0
Operating Segments | OEM
Segment Reporting Information [Line Items]
Sales												5,559	3,618	2,598
Income (loss) from operations												2,136	1,795	1,353
Interest income												0	0
Interest expense												0	0	0
Other losses, net												0	0
Change in fair value of derivative liabilities													0	0
Income tax benefit												0	0	0
Corporate (Other)
Segment Reporting Information [Line Items]
Sales												0	0	0
Income (loss) from operations												(14,960)	(9,745)	(11,281)
Interest income													616
Interest expense													(104)	(136)
Other losses, net													(947)
Change in fair value of derivative liabilities													20	183
Income tax benefit												$ 130	$ 3,907	$ 156