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Derivative Financial Instruments (Narrative) (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2018
USD ($)
agreement
Sep. 30, 2017
USD ($)
Derivative Financial Instruments [Abstract]    
Number of interest rate swap agreements 16  
Number of fuel hedge agreements 1  
Ineffectiveness recognized on the fuel hedges | $ $ 0 $ 0