Consolidated Statements of Operations (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
Collaboration revenues:
License and milestone revenue					$ 3,302	$ 1,143	$ 1,143	$ 1,143		$ 6,731	$ 4,572
Cost sharing reimbursements, net											9,253
Total collaboration revenues					3,302	1,143	1,143	1,143		6,731	13,825
Grant revenue				147	121	521	211		147	853	978
Total revenues				147	3,423	1,664	1,354	1,143	147	7,584	14,803
Operating expenses:
Research and development	14,351	11,743	11,252	12,066	10,859	10,751	10,417	9,436	49,412	41,464	40,252
General and administrative	3,352	2,796	2,882	2,646	2,803	3,131	2,946	2,673	11,676	11,552	11,449
Total operating expenses	17,703	14,539	14,134	14,712	13,662	13,882	13,363	12,109	61,088	53,016	51,701
Loss from operations	(17,703)	(14,539)	(14,134)	(14,565)	(10,239)	(12,218)	(12,009)	(10,966)	(60,941)	(45,432)	(36,898)
Interest expense, net	(420)	(457)	(486)	(486)	(504)	(516)	(493)	(435)	(1,849)	(1,948)	(569)
Net loss	$ (18,123)	$ (14,996)	$ (14,620)	$ (15,051)	$ (10,743)	$ (12,734)	$ (12,502)	$ (11,401)	$ (62,790)	$ (47,380)	$ (37,467)
Net loss per common share:
Basic and diluted net loss per common share (in dollars per share)	$ (0.29)	$ (0.25)	$ (0.25)	$ (0.27)	$ (0.22)	$ (0.26)	$ (0.26)	$ (0.27)	$ (1.06)	$ (1.00)	$ (0.93)
Basic and diluted weighted average number of common shares outstanding (in shares)	62,914,546	60,661,720	57,650,412	56,366,992	49,426,806	49,403,589	47,845,315	42,008,818	59,411,476	47,197,572	40,365,215