CASH, CASH EQUIVALENTS AND SHORT-TERM INVESTMENTS - Narrative (Details)		12 Months Ended
	Nov. 08, 2018 $ / shares	Dec. 31, 2018 USD ($) item $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	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 | item		0
Other than temporary impairments		$ 0
Gross realized gains and losses on marketable securities		$ 0	$ 0	$ 0
Convertible debt
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Interest rate		2.50%				2.50%
Par value of debt		$ 345,000,000				$ 345,000,000.0
Convertible debt | Fair Value | Level 2
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Estimated fair value of debt		$ 231,800,000
Lazanda
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Liabilities, contingent consideration					$ 200,000
Commercialization Agreement | Collegium Pharmaceutical Inc
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Warrant, fair value price (in dollars per share) | $ / shares	$ 15.56	$ 17.17
Collegium Pharmaceutical Inc | Commercialization Agreement
Schedule of Cash and Cash Equivalents and Marketable Securities [Line Items]
Warrant, expected term	4 years	3 years 10 months 10 days
Warrant, risk-free rate	3.05%	2.48%
Warrant, remaining contractual period		3 years 10 months 10 days