SELECTED QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SELECTED QUARTERLY FINANCIAL INFORMATION (UNAUDITED)
Revenues	$ 220,000	$ 35,363,000	$ 251,000	$ 537,000	$ 178,000	$ 160,000	$ 341,000	$ 474,000	$ 36,371,000	$ 1,153,000	$ 1,283,000
Operating expenses	8,864,000	8,343,000	9,730,000	9,993,000	9,189,000	10,067,000	9,004,000	9,205,000	36,930,000	37,465,000	40,241,000
Net (loss) income	$ (8,468,000)	$ 27,185,000	$ (9,356,000)	$ (9,315,000)	$ (8,947,000)	$ (9,549,000)	$ (8,734,000)	$ (8,440,000)	46,000	$ (35,670,000)	$ (38,379,000)
Basic and diluted net (loss) income per share	$ (0.05)	$ 0.17	$ (0.06)	$ (0.06)	$ (0.06)	$ (0.06)	$ (0.06)	$ (0.06)
Collaboration revenue		$ 35,000,000							$ 35,000,000