MARKETABLE SECURITIES (Schedule of Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value
Less Than 12 Months	$ 156,804	$ 100,024
12 Months or Greater	0	0
Total	156,804	100,024
Unrealized Loss
Less Than 12 Months	(159)	(56)
12 Months or Greater	0	0
Total	(159)	(56)
Agency bonds
Fair Value
Less Than 12 Months	78,913	92,018
12 Months or Greater	0	0
Total	78,913	92,018
Unrealized Loss
Less Than 12 Months	(45)	(53)
12 Months or Greater	0	0
Total	(45)	(53)
Corporate bonds
Fair Value
Less Than 12 Months	62,927	8,006
12 Months or Greater	0	0
Total	62,927	8,006
Unrealized Loss
Less Than 12 Months	(112)	(3)
12 Months or Greater	0	0
Total	(112)	$ (3)
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	3,975
12 Months or Greater	0
Total	3,975
Unrealized Loss
Less Than 12 Months	(1)
12 Months or Greater	0
Total	$ (1)