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Derivative Financial Instruments (Narrative) (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2017
USD ($)
agreement
Mar. 31, 2016
USD ($)
Derivative Financial Instruments [Abstract]    
Number of interest rate swap agreements 11  
Number of fuel hedge agreements 4  
Ineffectiveness recognized on the fuel hedges | $ $ 0 $ 0