Summary of Cash and Cash Equivalents and Restricted Cash Balances (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 72,224	$ 53,380	$ 89,317
Restricted cash	113,091	$ 157,914	193,580
Total cash and cash equivalents and restricted cash	$ 185,315		$ 282,897

Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant