Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivative Instrument (Carrying Amounts and Fair Value of Current and Long-term Debt) (Details)

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Fair Value Measurements and Derivative Instrument (Carrying Amounts and Fair Value of Current and Long-term Debt) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Carrying Amount    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Current and long-term debt $ 1,382,048 $ 1,336,772
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Current and long-term debt $ 1,458,106 $ 1,393,865
Senior Notes | 2021 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Stated percentage 6.75%  
Senior Notes | 2025 Notes    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Stated percentage 6.875%