INVESTMENTS (Fair value due by period) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Amortized Cost
Due in less than one year	$ 359,796			$ 359,796		$ 281,303
Due in one to three years	195,418			195,418		100,797
Amortized Cost	555,214			555,214		382,100
Estimated Fair Value
Due in less than one year	360,247			360,247		280,959
Due in one to three years	196,559			196,559		100,765
Estimated Fair Value	556,806			556,806		381,724
Equity investments in privately-held companies	35,496			35,496		$ 40,300
Gain on sale of investment		$ 9,100
Escrow deposits				3,200
Impairment charge	$ 0	$ 1,800	$ 0	$ 1,755	$ 0