Nature of Business - Additional Information (Detail) - USD ($) $ 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	Dec. 31, 2018
Nature Of Business [Abstract]
Net loss	$ 19,583	$ 19,426	$ 18,959	$ 15,884	$ 17,199	$ 20,588	$ 57,968	$ 53,671
Accumulated deficit	$ 730,894						$ 730,894		$ 672,926
Merger agreement date							Oct. 15, 2019