Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 57,960	$ 36,088	$ 33,642	$ 54,539
Restricted cash	1,513	1,513	1,513	1,400
Total cash, cash equivalents and restricted cash as shown in the statement of cash flows	$ 59,473	$ 37,601	$ 35,155	$ 55,939