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Accounting Policies (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Dec. 31, 2014
Revenue Recognition          
External freight billed to customers included in revenue $ 6,300 $ 6,800 $ 11,500 $ 13,400  
Recurring | Fair value | Cash flows approach          
Fair Values of Financial Instruments          
Interest rate swap liabilities         $ (661)
Recurring | Significant Other Observable Inputs (Level 2) | Cash flows approach          
Fair Values of Financial Instruments          
Interest rate swap liabilities         $ (661)