MARKETABLE SECURITITES Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 213,117	$ 188,543
Gross Unrealized Gain	47	38
Gross Unrealized Losses	(122)	(258)
Estimated Fair Market Value	213,042	188,323
Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,753	46,338
Gross Unrealized Gain	1	2
Gross Unrealized Losses	(2)	(2)
Estimated Fair Market Value	28,752	46,338
Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	148,140	78,900
Gross Unrealized Gain	28	15
Gross Unrealized Losses	(83)	(76)
Estimated Fair Market Value	148,085	78,839
Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,224	63,305
Gross Unrealized Gain	18	21
Gross Unrealized Losses	(37)	(180)
Estimated Fair Market Value	36,205	63,146
Corporate securities | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,278	9,342
Gross Unrealized Gain	1	2
Gross Unrealized Losses	(2)	(2)
Estimated Fair Market Value	19,277	9,342
Corporate securities | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	138,803	59,714
Gross Unrealized Gain	28	15
Gross Unrealized Losses	(78)	(76)
Estimated Fair Market Value	138,753	59,653
Corporate securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,224	63,305
Gross Unrealized Gain	18	21
Gross Unrealized Losses	(37)	(180)
Estimated Fair Market Value	36,205	63,146
Commercial paper | Investments due in 3 months or less:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,475	36,996
Gross Unrealized Gain	0	0
Gross Unrealized Losses	0	0
Estimated Fair Market Value	9,475	36,996
Commercial paper | Investments due in 4-12 months:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		19,186
Gross Unrealized Gain		0
Gross Unrealized Losses		0
Estimated Fair Market Value		$ 19,186
Government securities | Investments due in 12 months or greater:
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	9,337
Gross Unrealized Gain	0
Gross Unrealized Losses	(5)
Estimated Fair Market Value	$ 9,332