NATURE OF BUSINESS AND OVERVIEW (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) item	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
NATURE OF BUSINESS AND OVERVIEW
Number of drug candidates in clinical development | item	5								5
Cash, cash equivalents and marketable securities	$ 289,900								$ 289,900
Net Loss	$ (32,684)	$ (31,980)	$ (32,359)	$ (30,174)	$ (31,821)	$ (28,082)	$ (28,274)	$ (29,903)	(127,197)	$ (118,080)	$ (81,550)
Net cash used in operations									$ (98,887)	$ (101,537)	$ (67,669)