Significant Accounting Policies - Schedule of Effect of Change in Accounting Policy and its Impact on Key Components on Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Operating expenses:
Research and development	$ 3,049			$ 3,467			$ 11,464	$ 7,909
General and administrative	3,494			2,865			10,626	5,781
Total operating expenses	6,543			6,332			22,090	13,690
Loss from operations	(6,543)			(6,332)			(22,090)	(13,690)
Total other income	453			426			1,520	620
Net loss	$ (6,090)	$ (7,001)	$ (7,479)	$ (5,906)	$ (5,317)	$ (1,847)	$ (20,570)	$ (13,070)
Net loss per share, basic and diluted	$ (0.24)			$ (0.24)			$ (0.83)	$ (1.16)
Weighted average common shares outstanding, basic and diluted	24,893,036			24,847,512			24,875,589	11,230,401
As Previously Reported Under Graded Attribution Method [Member]
Operating expenses:
Research and development				$ 3,484				$ 8,022
General and administrative				3,681				7,170
Total operating expenses				7,165				15,192
Loss from operations				(7,165)				(15,192)
Total other income				426				620
Net loss				$ (6,739)				$ (14,572)
Net loss per share, basic and diluted				$ (0.27)				$ (1.30)
Weighted average common shares outstanding, basic and diluted				24,847,512				11,230,401