Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 3,431	$ 1,635
Prepaid R&D clinical costs	2,532	1,287
Prepaid insurance	706	124
Other prepaid costs	$ 193	$ 224