Supplemental Cash Flow Information of the Company - Reconciliation of cash and cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 86,517	$ 57,649	$ 64,954	$ 193,418
Restricted cash	0	9,149	179,276	56,711
Cash and cash equivalents and restricted cash	$ 86,517	$ 66,798	$ 244,230	$ 250,129

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