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Fair Value Disclosures (Estimated Fair Values and Carrying Amounts of Assets and Liabilities) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cash and cash equivalents, carrying value $ 314,094 $ 567,599
Cash and cash equivalents, fair value 314,094 567,599
Restricted cash, carrying value 126,065 129,547
Restricted cash, fair value 126,065 129,547
Debt and other obligations, carrying amount 16,159,620 12,171,142
Debt and other obligations, fair value $ 16,643,725 $ 12,660,013