Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 11,877	$ 9,388
Restricted cash, long-term assets	769	769
Total cash, cash equivalents, and restricted cash shown in the Condensed Statements of Cash Flows	$ 12,646	$ 10,157	$ 6,775	$ 13,561