Business Combinations - Activity (Details) - item	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
The Car People
Business combinations
Number of retail locations acquired	4
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	4	4

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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