Prepaid Expenses and Other Current Assets - Prepaid Expenses and Other Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Prepaid Expense And Other Assets Current [Abstract]
Prepaid insurance	$ 480	$ 1,518
Prepaid other	291	161
Other current assets	15	18
Total prepaid expenses and other current assets	$ 786	$ 1,697