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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration revenue	$ 274,730	$ 200,478	$ 148,090	$ 87,189	$ 96,612	$ 60,027	$ 70,149	$ 46,154	$ 710,487	$ 272,942	$ 181,696
Operating expenses	(131,253)	(108,595)	(93,139)	(95,654)	(88,587)	(68,025)	(70,095)	(68,476)	(428,641)	(295,183)	(207,910)
Income (loss) from operations	143,477	91,883	54,951	(8,465)	8,025	(7,998)	54	(22,322)	281,846	(22,241)	(26,214)
Net income (loss)	$ 164,205	$ 77,993	$ 47,919	$ (13,665)	$ 2,768	$ (13,313)	$ (4,898)	$ (27,170)	$ 276,452	$ (42,613)	$ (41,257)
Basic net income (loss) per common share	$ 2.11	$ 1.01	$ 0.63	$ (0.18)	$ 0.04	$ (0.18)	$ (0.07)	$ (0.36)	$ 3.59	$ (0.57)	$ (0.56)
Diluted net income (loss) per common share	$ 1.96	$ 0.96	$ 0.60	$ (0.18)	$ 0.03	$ (0.18)	$ (0.07)	$ (0.36)	$ 3.42	$ (0.57)	$ (0.56)
Weighted-average common shares used in the calculation of basic net income (loss) per common share	77,822	77,056	76,577	76,245	75,560	75,255	75,013	74,824	76,929	75,165	73,480
Weighted-average common shares used in the calculation of diluted net income (loss) per common share	85,756	81,223	80,491	76,245	79,912	75,255	75,013	74,824	85,000	75,165	73,480