MARKETABLE SECURITIES (Schedule of Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 250,247	$ 156,804
12 Months or Greater	0	0
Total	250,247	156,804
Unrealized Loss
Less Than 12 Months	(274)	(159)
12 Months or Greater	0	0
Total	(274)	(159)
Corporate bonds
Fair Value
Less Than 12 Months	132,992	62,927
12 Months or Greater	0	0
Total	132,992	62,927
Unrealized Loss
Less Than 12 Months	(174)	(112)
12 Months or Greater	0	0
Total	(174)	(112)
Agency bonds
Fair Value
Less Than 12 Months	48,047	78,913
12 Months or Greater	0	0
Total	48,047	78,913
Unrealized Loss
Less Than 12 Months	(67)	(45)
12 Months or Greater	0	0
Total	(67)	(45)
U.S. government bonds
Fair Value
Less Than 12 Months	69,208
12 Months or Greater	0
Total	69,208
Unrealized Loss
Less Than 12 Months	(33)
12 Months or Greater	0
Total	(33)
Agency discount notes
Fair Value
Less Than 12 Months		10,989
12 Months or Greater		0
Total		10,989
Unrealized Loss
Less Than 12 Months		(1)
12 Months or Greater		0
Total		(1)
Commercial paper
Fair Value
Less Than 12 Months	0	3,975
12 Months or Greater	0	0
Total	0	3,975
Unrealized Loss
Less Than 12 Months	0	(1)
12 Months or Greater	0	0
Total	$ 0	$ (1)