MARKETABLE SECURITIES (Tables)

12 Months Ended

Dec. 31, 2017

Marketable Securities [Abstract]

Available-for-sale Securities [Table Text Block]

Amortized cost and estimated fair market value of marketable securities classified as available-for-sale (excluding cash equivalents) at December 31, 2017, were as follows:

	Amortized		Gross Unrealized					Estimated Fair
(in thousands)	Cost		Gains		Losses			Market Value
Investments due in 3 months or less:
Corporate securities	$	38,485	$	—	$	(16	)	$	38,469
Total	38,485		—		(16		)	38,469
Investments due in 4-12 months:
Corporate securities	104,440		—		(199		)	104,241
Government securities	9,302		—		(17		)	9,285
Total	113,742		—		(216		)	113,526
Investments due in 12 months or greater:
Corporate securities	37,436		—		(195		)	37,241
Total	37,436		—		(195		)	37,241
Total marketable securities	$	189,663	$	—	$	(427	)	$	189,236

Amortized cost and estimated fair market value of marketable securities classified as available-for-sale (excluding cash equivalents) at December 31, 2016, were as follows:

	Amortized		Gross Unrealized					Estimated Fair
(in thousands)	Cost		Gains		Losses			Market Value
Investments due in 3 months or less:
Commercial paper	$	36,996	$	—	$	—		$	36,996
Corporate securities	9,342		2		(2		)	9,342
Total	46,338		2		(2		)	46,338
Investments due in 4-12 months:
Commercial paper	19,186		—		—			19,186
Corporate securities	59,714		15		(76		)	59,653
Total	78,900		15		(76		)	78,839
Investments due in 12 months or greater:
Corporate securities	63,305		21		(180		)	63,146
Total	63,305		21		(180		)	63,146
Total marketable securities	$	188,543	$	38	$	(258	)	$	188,323