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.

No definition available.

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

Business Combinations - Activity (Details) - item	6 Months Ended
Business Combinations - Activity (Details) - item	Jun. 30, 2018	Jun. 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	6
Retail Commercial Truck Dealership
Business combinations
Number of retail locations acquired	1