Summary of Significant Accounting Policies - Reconciliation of Amounts From Cash Flow Statement to Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Beginning And End Of Period Amounts From Cash Flow Statement To Balance Sheet [Abstract]
Cash and cash equivalents as presented on balance sheet	$ 86,366	$ 52,080	$ 7,678
Restricted cash, current, as presented on balance sheet		65	165
Restricted cash, non-current, as presented on balance sheet	327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$ 86,693	$ 52,145	$ 7,843	$ 28,595