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Receivables and Allowance for Doubtful Accounts Receivables and Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Receivables [Line Items]        
Accounts Receivable, Gross $ 73,066   $ 73,990  
Receivables, net of allowance for doubtful accounts of $3,388 and $4,167 in 2018 and 2017, respectively 69,678   69,823  
Allowance for Doubtful Accounts Receivable [Roll Forward]        
Bad debt expense 5,846 $ 6,395 15,702  
Accounts written off (6,959)   (15,791)  
Recoveries 334   663  
Allowance for doubtful accounts 3,388   4,167 $ 3,593
Installment sales receivable [Member]        
Receivables [Line Items]        
Accounts Receivable, Gross 53,112   55,516  
Allowance for Doubtful Accounts Receivable [Roll Forward]        
Allowance for doubtful accounts 2,800   3,600  
Trade and notes receivables [Member]        
Receivables [Line Items]        
Accounts Receivable, Gross 19,954   18,474  
Allowance for Doubtful Accounts Receivable [Roll Forward]        
Allowance for doubtful accounts $ 600   $ 600