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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 |

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