Summary of Significant Accounting Policies - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Money market funds, fair value	$ 3,933	$ 5,547
Cash Equivalents, fair value	3,933
Liabilities:
Warrant liability, fair value	2,155	14,679
Asset Backed Securities [Member]
Assets:
Marketable securities, fair value		900
Quoted Prices in Active Markets (Level 1) [Member]
Assets:
Money market funds, fair value	3,933	5,547
Cash Equivalents, fair value	3,933
Significant Other Observable Inputs (Level 2) [Member] | Asset Backed Securities [Member]
Assets:
Marketable securities, fair value		900
Significant Unobservable Inputs (Level 3) [Member]
Liabilities:
Warrant liability, fair value	$ 2,155	$ 14,679