Impact of Topic 606 Adoption on Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 02, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	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														35,139
Accumulated deficit		$ (764,101)				$ (684,125)				$ (764,101)	$ (684,125)			(712,442)
Collaboration revenue		0	$ 0	$ 0	$ 0							$ 146
Net loss		$ (22,772)	$ (20,315)	$ (18,596)	$ (18,293)	$ (15,746)	$ (73,996)	$ (14,620)	$ (13,434)	(79,976)	(117,796)	(53,117)
Net income (loss) applicable to common shareholders										$ (79,976)	$ (117,796)	$ 137,246
Net income (loss) per share—basic		$ (0.11)	$ (0.10)	$ (0.09)	$ (0.09)	$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ (0.38)	$ (0.70)	$ 0.96
Net income (loss) per share—diluted		$ (0.11)	$ (0.10)	$ (0.09)	$ (0.09)	$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ (0.38)	$ (0.70)	$ 0.96
Net loss		$ (22,772)	$ (20,315)	$ (18,596)	$ (18,293)	$ (15,746)	$ (73,996)	$ (14,620)	$ (13,434)	$ (79,976)	$ (117,796)	$ (53,117)
Changes in contract liability												(146)
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
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit													$ 8,100
Changes in contract liability	$ 8,100
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														(5,767)
Contract liability, net of current portion														13,898
Accumulated deficit														$ (8,131)
Collaboration revenue												(4,732)
Net loss												(4,732)
Net income (loss) applicable to common shareholders												$ (4,732)
Net income (loss) per share—basic												$ (0.03)
Net income (loss) per share—diluted												$ (0.03)
Net loss												$ (4,732)
Changes in contract liability											$ 0	0
Calculated under Revenue Guidance in Effect before Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue												4,878
Net loss												(48,385)
Net income (loss) applicable to common shareholders												$ 141,978
Net income (loss) per share—basic												$ 0.99
Net income (loss) per share—diluted												$ 0.99
Net loss												$ (48,385)