Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cash and cash equivalents	$ 5,192	$ 7,081
Restricted cash	100	$ 100
ASU 2016-18 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cash and cash equivalents	5,192		$ 10,798
Restricted cash	137		137
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 5,329		$ 10,935