CONCENTRATION OF RISK - Revenues by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	[1],[2]	Sep. 30, 2017	Jun. 30, 2017	[1]	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Revenues	$ 70,143	$ 77,550	$ 56,149	$ 43,572	$ 115,867		$ 61,683	$ 102,513		$ 55,566	$ 247,414	$ 335,629	$ 198,886
South Korea [Member]
Concentration Risk [Line Items]
Revenues											171,915	289,418	181,771
China [Member]
Concentration Risk [Line Items]
Revenues											51,931	24,892	7,180
Japan [Member]
Concentration Risk [Line Items]
Revenues											6,823	8,542	4,310
Other Non-U.S. Locations [Member]
Concentration Risk [Line Items]
Revenues											2,967	2,438	1,849
Total Non-U.S. Locations [Member]
Concentration Risk [Line Items]
Revenues											233,636	325,290	195,110
UNITED STATES
Concentration Risk [Line Items]
Revenues											$ 13,778	$ 10,339	$ 3,776

[1] Prior to the adoption of ASC 606 on January 1, 2018, the Company recorded as revenue the receipt of the semi-annual license fee payment of $45.0 million from SDC in the second and fourth quarters.

[2] The enactment of the Tax Cuts and Jobs Act in December 2017 resulted in a one-time charge of $11.5 million in the fourth quarter.