Prepaids and Other Current Assets - Schedule of Prepaids and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepaid Expense And Other Assets Current [Abstract]
Prepaid clinical, manufacturing and scientific expenses	$ 568	$ 1,390
Prepaid insurance expenses	55	104
Other prepaid expenses and other current assets	754	2,565
Prepaids and other current assets	$ 1,377	$ 4,059