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Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Schedule of Equity Method Investments [Line Items] |  |  |
Percentage of dealership that is accounted for under the equity method | 50.00% |  | 50.00%
Effective tax rate from continuing operations | 47.80% | 113.30% |
Discrete benefit related to vested or exercised stock compensation | $ 0.9 | $ 0.3 |
Effective Income Tax Rate Reconciliation, Uncertain Tax Position, Amount | $ 0.2 | $ 0.6 |
Minimum |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Expected annual effective tax rate in future periods | 26.00% |  |
Maximum |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Expected annual effective tax rate in future periods | 28.00% |  |
Dealership |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Percentage of dealership that is accounted for under the equity method | 50.00% |  |

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