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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Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Business Combinations - Activity (Details) - item	9 Months Ended
Business Combinations - Activity (Details) - item	Sep. 30, 2018	Sep. 30, 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	8
Retail Commercial Truck Dealership
Business combinations
Number of retail locations acquired	1