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 Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(2))
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Financial Instruments - Cash and Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 91,596	$ 103,828
Cash, cash equivalents and restricted cash	149,872	180,612	$ 213,231	$ 217,915
Amortized Cost
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	91,596	103,828
Restricted cash	58,276	76,784
Cash, cash equivalents and restricted cash	149,872	180,612
Amortized Cost \| Cash
Debt Securities, Available-for-sale [Line Items]
Cash, cash equivalents and restricted cash	58,492	101,356
Amortized Cost \| Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash, cash equivalents and restricted cash	91,380	79,256
Fair Value
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	91,596	103,828
Restricted cash	58,276	76,784
Cash, cash equivalents and restricted cash	149,872	180,612
Fair Value \| Cash
Debt Securities, Available-for-sale [Line Items]
Cash, cash equivalents and restricted cash	58,492	101,356
Fair Value \| Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash, cash equivalents and restricted cash	$ 91,380	$ 79,256