Basis of Presentation and Significant Accounting Policies - Cash, cash equivalents, and restricted cash equivalents (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		$ 179,507	$ 170,345	$ 158,577	$ 164,313
Restricted cash equivalents		39,809	32,531	5,322	1,142
Total cash, cash equivalents, and restricted cash equivalents shown in the statement of cash flows	[1]	$ 219,316	$ 202,876	$ 163,899	$ 165,455

[1] Certain prior period amounts have been revised due to the implementation of ASU 2016-18. See Note 1 for details.