Summary of Quarterly Results (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 3,598	$ 15,158	$ 134	$ 134	$ 115	$ 873	$ 1,846	$ 15,289	$ 19,024	$ 18,123	$ 1,293
Restructuring			25	4,333	1,302	1,403	5,165	3,859	4,358	11,729	8,017
Operating expenses	9,721	6,691	4,730	5,950	11,806	13,569	14,146	17,322
Loss from operations	(6,123)	8,467	(4,621)	(10,149)	(12,993)	(14,099)	(17,465)	(5,892)	(12,426)	(50,449)	(103,904)
Other expense, net	(462)	(553)	(835)	(725)	(901)	(337)	(494)	(558)	(2,575)	(2,290)	(3,125)
Net loss	$ (6,585)	$ 7,914	$ (5,456)	$ (10,874)	$ (13,894)	$ (14,436)	$ (17,959)	$ (6,450)	$ (15,001)	$ (52,739)	$ (107,029)
Net loss per share—basic and diluted	$ (0.11)	$ 0.14	$ (0.10)	$ (0.21)	$ (0.27)	$ (0.28)	$ (0.35)	$ (0.12)	$ (0.27)	$ (1.01)	$ (2.10)