Annual report pursuant to Section 13 and 15(d)

Basis of Presentation and Significant Accounting Policies Classification of Certain Cash Receipts and Cash Payments and Restricted Cash (Details)

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Basis of Presentation and Significant Accounting Policies Classification of Certain Cash Receipts and Cash Payments and Restricted Cash (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Basis of Presentation and Significant Accounting Policies [Abstract]      
Restricted cash $ 635,836 $ 408,052 $ 262,562