Annual report pursuant to Section 13 and 15(d)

Schedule II - Valuation and Qualifying Accounts (Details)

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Schedule II - Valuation and Qualifying Accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Valuation and Qualifying Accounts Disclosure [Line Items]        
Valuation Allowances and Reserves, Balance $ 3,366 $ 1,996 $ 2,689 $ 2,836
Valuation Allowances and Reserves, Charged to Cost and Expense 4,947 2,276 4,837  
Valuation Allowances and Reserves, Deductions 3,577 2,969 4,984  
Allowance for Doubtful Accounts [Member]        
Valuation and Qualifying Accounts Disclosure [Line Items]        
Valuation Allowances and Reserves, Balance 2,542 1,309 1,310 2,155
Valuation Allowances and Reserves, Charged to Cost and Expense 1,814 258 33  
Valuation Allowances and Reserves, Deductions [1] 581 259 878  
Allowance for revenue adjustments [Member]        
Valuation and Qualifying Accounts Disclosure [Line Items]        
Valuation Allowances and Reserves, Balance [2] 464 405 1,095 408
Valuation Allowances and Reserves, Charged to Cost and Expense [2] 3,055 2,020 4,793  
Valuation Allowances and Reserves, Deductions [2],[3] 2,996 2,710 4,106  
Valuation Allowance of Deferred Tax Assets [Member]        
Valuation and Qualifying Accounts Disclosure [Line Items]        
Valuation Allowances and Reserves, Balance 360 282 284 $ 273
Valuation Allowances and Reserves, Charged to Cost and Expense $ 78 $ (2) $ 11  
[1] Represents uncollectible accounts written off, net of recoveries
[2] Represents an allowance for adjustments to accounts receivable due to disputed rates, accessorial charges and other aspects of previously billed shipments.
[3] Represents adjustments to billed accounts receivable