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Debt (Schedule of Long-Term Debt) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Long-term principal $ 576,485 $ 587,485
Debt issuance costs (2,321) (3,015)
Unamortized discounts (472) (729)
Total long-term debt (includes current portion) 573,319 583,988
Less current portion of long-term debt (14,263) (11,032)
Long-term debt 559,056 572,956
Interest Rate Swaps [Member]    
Debt Instrument [Line Items]    
Debt instrument (373) 247
Term Loans [Member]    
Debt Instrument [Line Items]    
Long-term principal 343,500 349,500
Senior Notes [Member]    
Debt Instrument [Line Items]    
Long-term principal 150,000 150,000
Revenue Bonds [Member]    
Debt Instrument [Line Items]    
Long-term principal 65,735 65,735
Medium Term Notes [Member]    
Debt Instrument [Line Items]    
Long-term principal $ 17,250 $ 22,250