Interim Financial Statements (Business and Basis of Presentation) (Details)	3 Months Ended
	Mar. 31, 2018 brand store franchises
Product Information [Line Items]
Owned and operated new vehicle franchises | franchises	336
Number of stores	325
Number of brands | brand	33
Percentage of new vehicle sales from core brands (percent)	93.00%
Dealerships [Member]
Product Information [Line Items]
Number of stores	244
Collision Centers [Member]
Product Information [Line Items]
Number of stores	76