Impact of Topic 606 Adoption on Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion	$ 476			$ 6,389
Contract liability, net of current portion	49,037			35,139
Accumulated deficit	(736,593)			$ (712,442)
Collaboration revenue	146	$ 1,597
Net loss applicable to common shareholders	$ (21,140)	$ (19,658)
Basic and diluted net loss per share	$ (0.15)	$ (0.15)
Net loss applicable to common shareholders	$ (21,140)	$ (19,658)
Changes in contract liability	(146)	$ (1,597)
ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion			$ 622
Contract liability, net of current portion			49,037
Accumulated deficit			(720,573)
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion	(5,913)		(5,767)
Contract liability, net of current portion	15,495		13,898
Accumulated deficit	9,582		$ (8,131)
Collaboration revenue	(1,451)
Net loss applicable to common shareholders	$ (1,451)
Basic and diluted net loss per share	$ (0.01)
Net loss applicable to common shareholders	$ (1,451)
Changes in contract liability	1,451
Calculated under Revenue Guidance in Effect before Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion	6,389
Contract liability, net of current portion	33,542
Accumulated deficit	(746,175)
Collaboration revenue	1,597
Net loss applicable to common shareholders	$ (19,689)
Basic and diluted net loss per share	$ (0.14)
Net loss applicable to common shareholders	$ (19,689)
Changes in contract liability	$ (1,597)