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Impact of Topic 606 Adoption on Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion	$ 49,513		$ 49,513			$ 6,389
Contract liability, net of current portion						35,139
Accumulated deficit	(761,209)		(761,209)			$ (712,442)
Collaboration revenue			146
Net loss	$ (12,585)	$ (12,701)	$ (40,636)	$ (41,050)
Basic and diluted net loss per share	$ (0.13)	$ (0.13)	$ (0.41)	$ (0.41)
Net loss	$ (12,585)	$ (12,701)	$ (40,636)	$ (41,050)
Changes in contract liability			(146)	$ (4,792)
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	12,777		12,777		(5,767)
Contract liability, net of current portion					13,898
Accumulated deficit	12,777		12,777		$ (8,131)
Collaboration revenue	(1,597)		(4,646)
Net loss	$ (1,597)		$ (4,646)
Basic and diluted net loss per share	$ (0.01)		$ (0.04)
Net loss	$ (1,597)		$ (4,646)
Changes in contract liability			4,792
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	36,736		36,736
Accumulated deficit	(748,432)		(748,432)
Collaboration revenue	1,597		4,792
Net loss	$ (10,988)		$ (35,990)
Basic and diluted net loss per share	$ (0.12)		$ (0.37)
Net loss	$ (10,988)		$ (35,990)
Changes in contract liability			$ (4,792)