Vehicle Service Agreement Liability A reconciliation of the changes in the vehicle service agreement liability (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Product Warranty Liability [Line Items]
Extended Product Warranty Accrual	$ 42,021	$ 39,721	$ 40,794	$ 38,713
Extended Product Warranty Accrual, Increase for Warranties Issued	11,166	10,015
Deferred Revenue, Revenue Recognized	(8,015)
Extended Product Warranty Accrual, Decrease for Payments	(2,950)	(2,716)
Standard Product Warranty Accrual, Increase (Decrease) for Preexisting Warranties	(165)	(122)
Vehicle service agreement fees [Member]
Product Warranty Liability [Line Items]
Extended Product Warranty Accrual	42,021		$ 40,794
Deferred Revenue, Revenue Recognized	(9,588)	(8,848)
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 2,764	$ 2,679