QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended								4 Months Ended	12 Months Ended																									304 Months Ended
	Dec. 31, 2011	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 1987	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2006	Dec. 31, 2005	Dec. 31, 2004	Dec. 31, 2003	Dec. 31, 2002	Dec. 31, 2001	Dec. 31, 2000	Dec. 31, 1999	Dec. 31, 1998	Dec. 31, 1997	Dec. 31, 1996	Dec. 31, 1995	Dec. 31, 1994	Dec. 31, 1993	Dec. 31, 1992	Dec. 31, 1991	Dec. 31, 1990	Dec. 31, 1989	Dec. 31, 1988	Dec. 31, 2012
Revenues and other income
Interest income		$ 2	$ 1	$ 0	$ 0	$ 1	$ 0	$ 1	$ 0		$ 3	$ 2	$ 0																							$ 16,302
Total revenues and other income		2	1	0	0	1	0	1	0		3	2	421																							48,008
Expenses:
Research and development		6,568	3,131	2,178	1,466	1,701	3,234	2,267	1,480		13,343	8,682	2,904																							195,259
General and administrative		1,479	1,453	922	973	1,032	726	1,418	635		4,827	3,811	2,285																							52,920
Total expenses		8,047	4,584	3,100	2,439	2,733	3,960	3,685	2,115		18,170	12,493	5,189																							248,567
Net loss	$ (191,735)	$ (8,045)	$ (4,583)	$ (3,100)	$ (2,439)	$ (2,732)	$ (3,960)	$ (3,684)	$ (2,115)	$ (28)	$ (18,167)	$ (12,491)	$ (4,768)	$ (27,234)	$ (25,202)	$ (13,700)	$ (14,195)	$ (7,391)	$ (3,697)	$ (3,329)	$ (3,882)	$ (839)	$ (11,155)	$ (11,952)	$ (12,316)	$ (13,174)	$ (9,470)	$ (4,287)	$ (3,970)	$ (2,532)	$ (1,583)	$ (1,820)	$ (1,426)	$ (967)	$ (327)	$ (209,902)
Loss per share - basic and diluted		$ (0.47)	$ (0.30)	$ (0.21)	$ (0.17)	$ (0.22)	$ (0.32)	$ (0.30)	$ (0.20)		$ (1.18)	$ (1.04)	$ (0.59)
Shares used in loss per share calculation		17,134	15,422	14,826	13,983	12,320	12,315	12,296	10,790