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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement [Abstract]
Collaboration revenue	$ 96,612	$ 60,027	$ 70,149	$ 46,154	$ 37,161	$ 64,798	$ 42,912	$ 36,825	$ 272,942	$ 181,696	$ 60,389
Operating expenses	(88,587)	(68,025)	(70,095)	(68,476)	(63,916)	(64,390)	(43,887)	(35,717)	(295,183)	(207,910)	(103,319)
Income (loss) from operations	8,025	(7,998)	54	(22,322)	(26,755)	408	(975)	1,108	(22,241)	(26,214)	(42,930)
Net income (loss)	$ 2,768	$ (13,313)	$ (4,898)	$ (27,170)	$ (31,660)	$ (4,537)	$ (5,500)	$ 440	$ (42,613)	$ (41,257)	$ (38,841)
Basic net income (loss) per common share	$ 0.04	$ (0.18)	$ (0.07)	$ (0.36)	$ (0.43)	$ (0.06)	$ (0.08)	$ 0.01
Diluted net income (loss) per common share	$ 0.03	$ (0.18)	$ (0.07)	$ (0.36)	$ (0.43)	$ (0.06)	$ (0.08)	$ 0.01
Weighted-average common shares used in the calculation of basic net income (loss) per common share	75,560	75,255	75,013	74,824	74,489	73,697	72,770	71,907
Weighted-average common shares used in the calculation of diluted net income (loss) per common share	79,912	75,255	75,013	74,824	74,489	73,697	72,770	77,299