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, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]        
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount $ 2,476 $ 3,366 $ 1,996 $ 2,689
Valuation Allowances and Reserves, Charged to Cost and Expense 174 1,892 256  
SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction 4,692 3,577 2,969  
SEC Schedule, 12-09, Allowance, Credit Loss [Member]        
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]        
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount 1,309 2,542 1,309 1,310
Valuation Allowances and Reserves, Charged to Cost and Expense 139 1,814 258  
SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction [1] 1,372 581 259  
Allowance for revenue adjustments [Member]        
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]        
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount 772 464 [2] 405 [2] 1,095 [2]
SEC Schedule, 12-09, Valuation Allowances and Reserves, Additions, Charge to Other Account 3,628 3,055 2,020  
SEC Schedule, 12-09, Valuation Allowances and Reserves, Deduction [3] 3,320 2,996 2,710  
SEC Schedule, 12-09, Valuation Allowance, Deferred Tax Asset [Member]        
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]        
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount 395 360 282 $ 284
Valuation Allowances and Reserves, Charged to Cost and Expense $ 35 $ 78 $ (2)  
[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