Summary of Significant Accounting Policies (cash, cash equivalents and restricted cash) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 88,475	$ 106,307
Restricted cash current	10,132	28,234
Restricted cash non-current	952	890
Cash, cash equivalents and restricted cash	$ 99,559	$ 135,431	$ 123,160	$ 179,257

Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as noncurrent. 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