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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
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		$ (684,125)				$ (566,329)				$ (684,125)	$ (566,329)	(712,442)
Collaboration revenue									$ 146		146	6,389
Net loss		$ (15,746)	$ (73,996)	$ (14,620)	$ (13,434)	$ 194,538	$ (18,659)	$ (17,493)	$ (21,140)	(117,796)	(53,117)	(54,323)
Net income (loss) applicable to common shareholders										$ (117,796)	$ 137,246	$ (73,261)
Net income (loss) per share—basic		$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)	$ (0.70)	$ 0.96	$ (0.53)
Net income (loss) per share—diluted		$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)	$ (0.70)	$ 0.96	$ (0.53)
Net loss		$ (15,746)	$ (73,996)	$ (14,620)	$ (13,434)	$ 194,538	$ (18,659)	$ (17,493)	$ (21,140)	$ (117,796)	$ (53,117)	$ (54,323)
Changes in contract liability											(146)	(6,389)
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)