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Long-Term Debt (Principal Outstanding) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Principal outstanding $ 640,508  
2027 Notes [Member]    
Debt Instrument [Line Items]    
Annual interest rate (as a percentage) 5.875%  
Principal outstanding $ 265,000 $ 0
2023 Senior Notes [Member]    
Debt Instrument [Line Items]    
Annual interest rate (as a percentage) 6.625%  
Principal outstanding $ 348,069 $ 393,767