Selected Quarterly Financial Data (Selected Quarterly Financial Data) (Detail) (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
Collaboration revenue	$ 37,161	$ 64,798	$ 42,912	$ 36,825	$ 14,941	$ 14,940	$ 15,799	$ 14,709	$ 181,696	$ 60,389	$ 62,508
Operating expenses	(63,916)	(64,390)	(43,887)	(35,717)	(26,940)	(26,430)	(26,175)	(23,774)	(207,910)	(103,319)	(95,233)
Income (loss) from operations	(26,755)	408	(975)	1,108	(11,999)	(11,490)	(10,376)	(9,065)	(26,214)	(42,930)	(32,725)
Net income (loss)	$ (31,660)	$ (4,537)	$ (5,500)	$ 440	$ (10,871)	$ (10,044)	$ (9,474)	$ (8,452)	$ (41,257)	$ (38,841)	$ (34,037)
Basic net income (loss) per common share	$ (0.43)	$ (0.06)	$ (0.08)	$ 0.01
Basic and diluted net loss per common share					$ (0.15)	$ (0.14)	$ (0.14)	$ (0.12)	$ (0.56)	$ (0.56)	$ (0.50)
Diluted net income (loss) per common share	$ (0.43)	$ (0.06)	$ (0.08)	$ 0.01
Weighted-average common shares used in the calculation of basic and diluted net loss per common share					70,749	69,818	69,776	69,325	73,480	69,921	68,579
Weighted-average common shares used in the calculation of basic net income (loss) per common share	74,489	73,697	72,770	71,907
Weighted-average common shares used in the calculation of diluted net income (loss) per common share	74,489	73,697	72,770	77,299