Business Combinations - Activity (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) item	Dec. 31, 2017 item
Business combinations
Revenue from acquisition \| $	$ 484.7
Pre-tax income from acquisition \| $	$ 14.1
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	6	8
Retail Commercial Truck Dealership
Business combinations
Number of dealerships acquired	1

This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

Name:	us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration