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Fair Value Disclosures (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents $ 54,846 $ 509,167 $ 291,535 $ 46,299
Long-Term Debt 2,330,780 2,187,289    
Long-term Debt, Fair Value 2,329,810 2,281,282    
Carrying Amount [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents 54,846 509,167    
Long-Term Debt $ 2,342,254 $ 2,204,825