Business Combinations - Activity (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($) item	Dec. 31, 2015 item
Business combinations
Revenue from acquisition \| $	$ 315.9
Pre-tax income from acquisition \| $	$ 0.5
Retail Automotive Dealership
Business combinations
Number of acquired franchises \| item	20	5
Retail Commercial Truck Dealership
Business combinations
Number of dealerships acquired \| item	7	2

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)
-URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463

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