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Acquisitions (Narrative) (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Mar. 01, 2018
store
Feb. 26, 2018
store
Aug. 07, 2017
store
May 01, 2017
store
Business Acquisition [Line Items]                
Acquisition expense recognized | $ $ 3.3 $ 2.1 $ 4.2 $ 2.2        
Day Auto Group                
Business Acquisition [Line Items]                
Number of stores acquired           7    
Prestige Auto Group                
Business Acquisition [Line Items]                
Number of stores acquired         6      
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