MARKETABLE SECURITIES (Schedule of Securities in an Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value
Fair Value, Less Than 12 Months	$ 132,941	$ 100,024
Fair Value, 12 Months or Greater	0	0
Fair Value, Total	132,941	100,024
Unrealized Loss
Unrealized Loss, Less Than 12 Months	(58)	(56)
Unrealized Loss, 12 Months or Greater	0	0
Unrealized Loss, Total	(58)	(56)
Agency bonds
Fair Value
Fair Value, Less Than 12 Months	122,948	92,018
Fair Value, 12 Months or Greater	0	0
Fair Value, Total	122,948	92,018
Unrealized Loss
Unrealized Loss, Less Than 12 Months	(53)	(53)
Unrealized Loss, 12 Months or Greater	0	0
Unrealized Loss, Total	(53)	(53)
Corporate bonds
Fair Value
Fair Value, Less Than 12 Months	9,993	8,006
Fair Value, 12 Months or Greater	0	0
Fair Value, Total	9,993	8,006
Unrealized Loss
Unrealized Loss, Less Than 12 Months	(5)	(3)
Unrealized Loss, 12 Months or Greater	0	0
Unrealized Loss, Total	$ (5)	$ (3)