Vehicle Service Agreement Liability A reconciliation of the changes in the vehicle service agreement liability, including deferred service fees related to vehicle service agreements (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Product Warranty Liability [Line Items]
Extended Product Warranty Accrual	$ 43,196	$ 40,005	$ 40,794	$ 38,713
Extended Product Warranty Accrual, Increase for Warranties Issued	17,247	14,129
Deferred Revenue, Revenue Recognized	(12,700)
Extended Product Warranty Accrual, Decrease for Payments	(4,378)	(4,129)
Standard Product Warranty Accrual, Increase (Decrease) for Preexisting Warranties	(252)	(139)
Vehicle service agreement fees [Member]
Product Warranty Liability [Line Items]
Extended Product Warranty Accrual	43,196		$ 40,794
Deferred Revenue, Revenue Recognized	(14,421)	(12,635)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 4,206	$ 4,066