Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per 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
Summary of quarterly financial data
Revenues	$ 8,931	$ 4,907	$ 4,574	$ 3,243	$ 4,748	$ 1,857	$ 1,514	$ 2,200	$ 21,655	$ 10,319	$ 20,805
Expenses	12,300	10,709	10,318	10,526	11,113	11,431	11,795	11,801	43,853	46,140	45,740
Net loss	$ (3,478)	$ (5,790)	$ (5,728)	$ (7,268)	$ (6,359)	$ (9,554)	$ (10,259)	$ (9,578)	$ (22,264)	$ (35,750)	$ (24,854)
Net loss per share	$ (0.07)	$ (0.11)	$ (0.11)	$ (0.13)	$ (0.12)	$ (0.18)	$ (0.20)	$ (0.21)	$ (0.42)	$ (0.71)	$ (0.55)