INTEREST AND OTHER, NET (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Income and Expenses [Abstract]
Interest expense			$ (149)	$ (2,185)	$ (7,937)
Interest income and gains on short-term investments, net			15,588	5,629	3,748
Foreign exchange loss, net			(7,070)	(1,256)	(596)
Gain on investments in privately-held companies		$ 0	9,569	0	0
Impairment of investments in privately-held companies	$ (1,800)	$ (1,500)	(1,755)	(1,494)	0
Other			(174)	(557)	(37)
Interest and other, net			$ 16,009	$ 137	$ (4,822)