MARKETABLE SECURITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Amortized Cost
Amortized cost	$ 190,901	$ 218,070
Gross Unrealized Gains
Gross Unrealized Gains	117	31
Gross Unrealized Losses
Gross Unrealized Losses	(26)	(320)
Fair Value
Fair Value	190,992	217,781
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	900	1,500
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	70,124	48,871
Maturing after one year through three years	5,396	15,940
Amortized cost	75,520	64,811
Gross Unrealized Gains
Maturing in one year or less	32	4
Maturing after one year through three years	31	24
Gross Unrealized Gains	63	28
Gross Unrealized Losses
Maturing in one year or less	(3)	(20)
Maturing after one year through three years		(57)
Gross Unrealized Losses	(3)	(77)
Fair Value
Maturing in one year or less	70,153	48,855
Maturing after one year through three years	5,427	15,907
Fair Value	75,580	64,762
Corporate debt securities
Amortized Cost
Maturing in one year or less	97,686	129,327
Maturing after one year through three years	17,695	23,932
Amortized cost	115,381	153,259
Gross Unrealized Gains
Maturing in one year or less	23	2
Maturing after one year through three years	31	1
Gross Unrealized Gains	54	3
Gross Unrealized Losses
Maturing in one year or less	(18)	(141)
Maturing after one year through three years	(5)	(102)
Gross Unrealized Losses	(23)	(243)
Fair Value
Maturing in one year or less	97,691	129,188
Maturing after one year through three years	17,721	23,831
Fair Value	$ 115,412	$ 153,019