THE COMPANY AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Equity investments in privately-held companies (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
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Impairment of investment in a privately-held company	$ 1,800	$ 1,500	$ 1,755	$ 1,494	$ 0