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Impact of Topic 606 Adoption on Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion						$ 6,389
Contract liability, net of current portion	$ 49,513		$ 49,513			35,139
Accumulated deficit	(748,624)		(748,624)			$ (712,442)
Collaboration revenue			146
Net loss	$ (12,031)	$ (12,862)	$ (28,051)	$ (28,349)
Basic and diluted net loss per share	$ (0.12)	$ (0.13)	$ (0.27)	$ (0.28)
Net loss	$ (12,031)	$ (12,862)	$ (28,051)	$ (28,349)
Changes in contract liability			(146)	$ (3,194)
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	(6,389)		(6,389)		(5,767)
Contract liability, net of current portion	17,569		17,569		13,898
Accumulated deficit	11,180		11,180		$ (8,131)
Collaboration revenue	(1,597)		(3,048)
Net loss	$ (1,597)		$ (3,048)
Basic and diluted net loss per share	$ (0.01)		$ (0.02)
Net loss	$ (1,597)		$ (3,048)
Changes in contract liability			3,194
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		6,389
Contract liability, net of current portion	31,944		31,944
Accumulated deficit	(737,444)		(737,444)
Collaboration revenue	1,597		3,194
Net loss	$ (10,434)		$ (25,003)
Basic and diluted net loss per share	$ (0.11)		$ (0.25)
Net loss	$ (10,434)		$ (25,003)
Changes in contract liability			$ (3,194)