Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	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, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Data (Unaudited) [Abstract]
Grant Revenues	$ 195	$ 0	$ 0	$ 0	$ 0	$ 0	$ 201	$ 381	$ 195	$ 582	$ 0
Expenses:
Research & development	6,088	5,743	5,206	4,533	4,014	3,981	4,615	4,620	21,570	17,230	17,136
General and administrative	6,532	4,255	3,610	2,047	1,889	2,189	1,966	1,820	16,444	7,864	8,392
Total expenses	12,620	9,998	8,816	6,580	5,903	6,170	6,581	6,440	38,014	25,094	25,528
Operating loss	(12,425)	(9,998)	(8,816)	(6,580)	(5,903)	(6,170)	(6,380)	(6,059)	(37,819)	(24,512)	(25,528)
Change in fair value of common stock warrant liability	5,618	(3,309)	1,680	(3,434)	1,603	1,422	(1,693)	2,228	555	3,560	6,422
Other expense, net	(8)	(39)	(2)	(2)	(1)	(3)	(3)	(6)	(51)	(13)	(69)
Net loss	$ (6,815)	$ (13,346)	$ (7,138)	$ (10,016)	$ (4,301)	$ (4,751)	$ (8,076)	$ (3,837)	$ (37,315)	$ (20,965)	$ (19,175)
Net loss per common share - basic and diluted (in dollars per share)	$ (0.16)	$ (0.31)	$ (0.16)	$ (0.37)	$ (0.18)	$ (0.2)	$ (0.34)	$ (0.21)	$ (0.95)	$ (0.93)	$ (1.65)
Weighted average number of common shares outstanding - basic and diluted (in shares)	43,521	43,444	43,369	27,162	24,309	24,106	24,027	18,114	39,396	22,660