Selected Quarterly Financial Data - Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Collaboration revenue	$ 377,748	$ 260,665	$ 175,657	$ 129,188	$ 274,730	$ 200,478	$ 148,090	$ 87,189	$ 943,258	$ 710,487	$ 272,942
Operating expenses	(156,348)	(121,680)	(122,002)	(128,615)	(131,253)	(108,595)	(93,139)	(95,654)	(528,645)	(428,641)	(295,183)
Income (loss) from operations	221,400	138,985	53,655	573	143,477	91,883	54,951	(8,465)	414,613	281,846	(22,241)
Net income (loss)	$ 142,507	$ 79,510	$ 25,826	$ (3,118)	$ 164,205	$ 77,993	$ 47,919	$ (13,665)	$ 244,725	$ 276,452	$ (42,613)
Basic net income (loss) per common share	$ 0.87	$ 0.49	$ 0.16	$ (0.02)	$ 1.06	$ 0.51	$ 0.31	$ (0.09)	$ 1.53	$ 1.80	$ (0.28)
Diluted net income (loss) per common share	$ 0.85	$ 0.47	$ 0.15	$ (0.02)	$ 0.98	$ 0.48	$ 0.30	$ (0.09)	$ 1.47	$ 1.71	$ (0.28)
Weighted-average common shares used in the calculation of basic net income (loss) per common share	163,746	162,390	158,505	156,637	155,644	154,112	153,153	152,489	160,345	153,859	150,331
Weighted-average common shares used in the calculation of diluted net income (loss) per common share	168,368	168,070	168,690	156,637	171,513	162,446	160,983	152,489	169,324	170,001	150,331