Receivables and Allowance for Doubtful Accounts Receivables and Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Receivables [Line Items]
Accounts Receivable, Gross	$ 69,444		$ 73,990
Receivables, net of allowance for doubtful accounts of $4,247 and $4,167 in 2018 and 2017, respectively	65,197		69,823
Allowance for Doubtful Accounts Receivable [Roll Forward]
Bad debt expense	10,372	$ 11,051	15,702
Accounts written off	(10,740)		(15,791)
Recoveries	448		663
Allowance for doubtful accounts	4,247		4,167	$ 3,593
Installment sales receivable [Member]
Receivables [Line Items]
Accounts Receivable, Gross	51,477		55,516
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for doubtful accounts	3,000		3,600
Trade and notes receivables [Member]
Receivables [Line Items]
Accounts Receivable, Gross	17,967		18,474
Allowance for Doubtful Accounts Receivable [Roll Forward]
Allowance for doubtful accounts	$ 1,200		$ 600