INVESTMENTS - Contractual maturities of short-term investments (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Due in less than one year			$ 281,303	$ 424,616	$ 281,303
Due in one to three years			100,797	372,094	100,797
Amortized Cost			382,100	796,710	382,100
Estimated Fair Value
Due in less than one year			280,959	425,053	280,959
Due in one to three years			100,765	373,265	100,765
Estimated fair value			381,724	798,318	381,724
Equity investments in privately-held companies			40,300	39,300	40,300
Gain on sale of investment		$ 9,100
Escrow deposits				3,200
Gain conversion of note receivable to equity	$ 400
Impairment of investment in a privately-held company		$ 1,800	$ 1,500	$ 1,755	$ 1,494	$ 0