Balance Sheet Components - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Components [Abstract]
Prepaid expenses	$ 1,823	$ 978
Accounts and other receivables	48	128
Litigation settlement receivable due from insurance	0	5,898
Other prepaid and current assets	444	163
Total prepaid expenses and other current assets	$ 2,315	$ 7,167