Consolidated Statements of Operations (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Statement [Abstract]
Collaboration revenue	$ 6,268	$ 6,268	$ 12,536	$ 12,536
Grant revenue		20		20
Total revenue	6,268	6,288	12,536	12,556
Operating expenses:
Research and development	12,958	10,463	24,420	20,014
General and administrative	1,836	1,948	3,688	3,930
Total operating expenses	14,794	12,411	28,108	23,944
Loss from operations	(8,526)	(6,123)	(15,572)	(11,388)
Interest income	5	22	13	48
Loss before (benefit) provision for income taxes	(8,521)	(6,101)	(15,559)	(11,340)
(Benefit) provision for income taxes	99	(588)	289	(1,196)
Net loss	(8,620)	(5,513)	(15,848)	(10,144)
Cumulative preferred stock dividends	(1,798)	(1,798)	(3,595)	(3,595)
Net loss applicable to common stockholders	$ (10,418)	$ (7,311)	$ (19,443)	$ (13,739)
Net loss per share applicable to common stockholders - basic and diluted	$ (2.80)	$ (2.18)	$ (5.27)	$ (4.16)
Weighted-average number of common shares used in net loss per share applicable to common stockholders - basic and diluted	3,722,963	3,358,172	3,690,669	3,302,508