Summary of Significant Accounting Policies - 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 Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 47,194	$ 36,088	$ 41,899	$ 54,539
Restricted cash	1,513	1,513	1,401	1,400
Total cash, cash equivalents and restricted cash as shown in the statement of cash flows	$ 48,707	$ 37,601	$ 43,300	$ 55,939