Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 4,218	$ 1,635
Prepaid R&D clinical costs	3,466	1,287
Prepaid insurance	505	124
Other prepaid costs	$ 247	$ 224