Summary of Significant Accounting Policies Supplemental Cash Flow Disclosure (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Elements [Abstract]
Cash and cash equivalents	$ 164,460	$ 330,039	$ 234,659
Current restricted cash	4,575	8,625
Restricted Cash and Cash Equivalents, Noncurrent	1,938	1,937
Restricted Cash and Cash Equivalents	6,513	10,562
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 170,973	$ 340,601	$ 234,659	$ 211,506

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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-SubTopic 210
-Section S99
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant