Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,635	$ 1,530
Prepaid R&D clinical costs	1,287	1,256
Prepaid insurance	124	112
Other prepaid costs	$ 224	$ 162