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Business Acquisitions and Dispositions - Additional Information (Detail) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) Dealerships Store Franchise | Dec. 31, 2016 USD ($) Dealerships Business | Dec. 31, 2015 USD ($) Dealerships Franchise
Business Acquisition [Line Items] |  |  |
Goodwill acquired related to acquisition | $ 60,024 |  |
Number of businesses acquired | Franchise |  |  | 0
Number of franchises disposed | Dealerships | 3 | 0 | 4
Cash generated from disposition | $ 38,200 |  | $ 8,000
Number of franchises classified as held for sale | Franchise | 0 |  |
EchoPark Stores [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Number of stores | Store | 2 |  |
Stand-Alone Pre-Owned Vehicle Store [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition, consideration amount | $ 76,600 |  |
Business acquisition, deferred payment | $ 10,000 |  |
Business acquisition, deferred payment term | 2 years |  |
Business acquisition, additional variable payment term | 10 years |  |
Business combination, accrued for deferred and variable payments | $ 1,300 |  |
Stand-Alone Pre-Owned Vehicle Store [Member] | Maximum [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition, additional variable payment | 80,000 |  |
Stand-Alone Pre-Owned Vehicle Store [Member] | Maximum [Member] | Employee Termination [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition, additional variable payment | 80,000 |  |
Stand-Alone Pre-Owned Vehicle Store [Member] | Pre-Owned Stores Segment [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Goodwill acquired related to acquisition | $ 60,000 |  |
Stand-Alone Pre-Owned Vehicle Store And Related Real Estate [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Business acquisition, consideration amount |  | $ 15,900 |
Number of businesses acquired | Business |  | 3 |

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