Vehicle Financing (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Vehicle Financing
Maturity period of floor plan arrangements outside the U.S. if not payable on demand	90 days
Weighted average interest rate on floor plan borrowings (as a percent)	1.80%	1.50%	1.50%