CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	Sep. 09, 2014 USD ($)
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Number of securities in an unrealized loss position (security) | security	0		0
Other than temporary impairments			$ 0		$ 0
Gross realized gains and losses on marketable securities	$ 0	$ 0	0	$ 0
Convertible debt
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Interest rate							2.50%
Par value of debt							$ 345,000,000
Convertible debt | Fair Value | Level 2
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Estimated fair value of debt	$ 276,900,000		$ 276,900,000		$ 295,400,000
Lazanda
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Liabilities, contingent consideration						$ 200,000