Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 5,615	$ 1,635
Prepaid R&D clinical costs	4,923	1,287
Prepaid insurance	479	124
Other prepaid costs	$ 213	$ 224