This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e1043-128460

Name:	us-gaap_BusinessCombinationAcquisitionRelatedCosts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Acquisitions (Narrative) (Details) $ in Millions	12 Months Ended
Acquisitions (Narrative) (Details) $ in Millions	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 07, 2017 store	May 01, 2017 store	Sep. 12, 2016 store
Business Acquisition [Line Items]
Acquisition expenses \| $	$ 6.0	$ 1.0
Baierl Auto Group
Business Acquisition [Line Items]
Number of stores acquired				8
Downtown LA Auto Group
Business Acquisition [Line Items]
Number of stores acquired			7
Carbone Auto Group
Business Acquisition [Line Items]
Number of stores acquired					9