MARKETABLE SECURITIES (Schedule of Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value
Less Than 12 Months	$ 228,008	$ 156,804
12 Months or Greater	0	0
Total	228,008	156,804
Unrealized Loss
Less Than 12 Months	(421)	(159)
12 Months or Greater	0	0
Total	(421)	(159)
Corporate bonds
Fair Value
Less Than 12 Months	89,672	62,927
12 Months or Greater	0	0
Total	89,672	62,927
Unrealized Loss
Less Than 12 Months	(291)	(112)
12 Months or Greater	0	0
Total	(291)	(112)
Agency bonds
Fair Value
Less Than 12 Months	80,861	78,913
12 Months or Greater	0	0
Total	80,861	78,913
Unrealized Loss
Less Than 12 Months	(87)	(45)
12 Months or Greater	0	0
Total	(87)	(45)
U.S. government bonds
Fair Value
Less Than 12 Months	38,609
12 Months or Greater	0
Total	38,609
Unrealized Loss
Less Than 12 Months	(31)
12 Months or Greater	0
Total	(31)
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	18,866	3,975
12 Months or Greater	0	0
Total	18,866	3,975
Unrealized Loss
Less Than 12 Months	(12)	(1)
12 Months or Greater	0	0
Total	$ (12)	$ (1)