Summary of Significant Accounting Policies - Summary of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 241,569	$ 52,026	$ 62,867
Restricted cash	175,665	87,616	92,475
Total cash, cash equivalents, and restricted cash shown in the statement of cash flows	$ 417,234	$ 139,642	$ 155,342