Selected Quarterly Financial Data (Selected Quarterly Financial Data) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Data [Abstract]
Collaboration revenue	$ 14,941	$ 14,940	$ 15,799	$ 14,709	$ 16,632	$ 14,350	$ 15,792	$ 15,734	$ 60,389	$ 62,508	$ 69,254
Operating expenses	(26,940)	(26,430)	(26,175)	(23,774)	(17,496)	(21,087)	(23,229)	(33,421)	(103,319)	(95,233)	(116,711)
Loss from operations	(11,999)	(11,490)	(10,376)	(9,065)	(864)	(6,737)	(7,437)	(17,687)	(42,930)	(32,725)	(47,457)
Net loss	$ (10,871)	$ (10,044)	$ (9,474)	$ (8,452)	$ (3,889)	$ (5,443)	$ (7,240)	$ (17,465)	$ (38,841)	$ (34,037)	$ (54,753)
Basic and diluted net loss per common share	$ (0.31)	$ (0.29)	$ (0.27)	$ (0.24)	$ (0.11)	$ (0.16)	$ (0.21)	$ (0.51)	$ (1.11)	$ (0.99)	$ (1.71)
Weighted-average common shares used in the calculation of basic and diluted net loss per common share	35,375	34,909	34,888	34,663	34,573	34,570	34,053	33,953	35,375	34,573