CONCENTRATION OF RISK - Revenues by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Revenues	$ 141,543	$ 117,079	$ 57,968	$ 112,277	$ 101,729	$ 97,515	$ 118,168	$ 87,765	$ 428,867	$ 405,177	$ 247,414
South Korea [Member]
Concentration Risk [Line Items]
Revenues									263,079	250,562	171,915
China [Member]
Concentration Risk [Line Items]
Revenues									142,076	135,259	51,931
Japan [Member]
Concentration Risk [Line Items]
Revenues									7,405	5,276	6,823
Other Non-U.S. Locations [Member]
Concentration Risk [Line Items]
Revenues									1,728	2,270	2,967
Total Non-U.S. Locations [Member]
Concentration Risk [Line Items]
Revenues									414,288	393,367	233,636
United States [Member]
Concentration Risk [Line Items]
Revenues									$ 14,579	$ 11,810	$ 13,778