O. SEGMENT AND GEOGRAPHIC INFORMATION - REVENUES (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Number of Reportable segment | item									1
Revenues	$ 79,855	$ 88,879	$ 117,371	$ 96,224	$ 84,900	$ 70,137	$ 55,254	$ 50,422	$ 382,329	$ 260,713	$ 189,903
UNITED STATES
Revenues									18,209	18,035	16,766
TAIWAN, PROVINCE OF CHINA
Revenues									241,820	161,611	141,107
CHINA
Revenues									$ 122,300	$ 81,067	$ 32,030