MARKETABLE SECURITIES - Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Marketable securities	$ 202,939	$ 201,236
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Marketable securities	10,339	6,897
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Marketable securities	162,648	164,297
Securities of government sponsored entities
Schedule of Available-for-sale Securities [Line Items]
Marketable securities	$ 29,952	$ 30,042