Nature of the Business - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Net loss	$ (46,361)	$ (18,648)
Accumulated deficit	(71,739)	(23,474)
Cash and cash equivalents	$ 126,302	$ 5,963