Business Combinations - Activity (Details) - item	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
CarSense
Business combinations
Number of retail locations acquired	5
CarShop
Business combinations
Number of retail locations acquired	5
Retail Automotive Dealership
Business combinations
Number of acquired franchises	8	13
Retail Commercial Truck Dealership
Business combinations
Number of retail locations acquired		5

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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