Quarterly Consolidated Financial Data (Unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended								24 Months Ended	98 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2006	Dec. 31, 2012	Dec. 31, 2013
Costs and expenses:
Research and Development	$ 16,449	$ 10,782	$ 9,055	$ 14,344	$ 9,085	$ 7,245	$ 7,626	$ 5,338	$ 50,630	$ 29,294	$ 13,419							$ 108,337
Purchased in-process research and development						1,000				1,000								29,157
General and administrative	2,908	2,692	2,803	3,278	2,447	1,843	1,919	1,732	11,681	7,941	6,745							39,266
Loss from Operations	(19,357)	(13,474)	(11,858)	(17,622)	(11,532)	(10,088)	(9,545)	(7,070)	(62,311)	(38,235)	(20,164)							(176,760)
Other income					250		256			506	362						868	1,363
Interest and investment income	11	14	16	17	68	63	48	39	59	218	90							555
Interest expense	(21)								(21)		(12)							(33)
Change in fair value of derivative instruments-warrants	(484)	(77)	1,803	(1,093)	(764)	140	(1,317)	8	149	(1,933)	5,257							3,770
Total Other Income/(loss)	(494)	(63)	1,819	(1,076)	(446)	203	(1,013)	47	187	(1,209)	5,697							5,655
Net loss	$ (19,851)	$ (13,537)	$ (10,039)	$ (18,698)	$ (11,978)	$ (9,885)	$ (10,558)	$ (7,023)	$ (62,124)	$ (39,444)	$ (14,467)	$ (15,221)	$ (8,124)	$ (31,757)	$ (20)	$ (20)		$ (171,177)
Weighted average common shares outstanding-basic and diluted (in shares)	90,182,115	90,182,115	87,482,939	72,789,006	66,194,306	65,806,178	60,416,068	54,298,079	85,220,458	61,702,277	47,598,240
Net loss per common share-basic and diluted (in dollars per share)	$ (0.22)	$ (0.15)	$ (0.11)	$ (0.26)	$ (0.18)	$ (0.15)	$ (0.17)	$ (0.13)	$ (0.73)	$ (0.64)	$ (0.30)