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Accounting Policies (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Revenue Recognition          
External freight billed to customers included in revenue $ 6,400 $ 7,000 $ 17,900 $ 20,400  
Recurring | 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)