Organization and Description of Business - Additional Information (Detail) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) Segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of operating segments | Segment							1
Net loss	$ (11,421)	$ (10,726)	$ (13,088)	$ (26,282)	$ (24,038)	$ (23,075)	$ (35,235)	$ (73,395)
Cash flows from operating activities							(29,909)	$ (69,775)
Accumulated deficit	(661,107)						(661,107)		$ (625,872)
Cash and cash equivalents and marketable securities	$ 161,300						$ 161,300