| Summary of Significant Accounting Policies - Summary of Lease Exit Accruals (Parenthetical) (Detail) $ in Millions | 6 Months Ended | 
|---|---|
| Jun. 30, 2016  USD ($) | |
| Accounting Policies [Abstract] | |
| Component of lease exit income in income (loss) from operations and the sale of dealerships before tax | $ 0.2 | 
| Component of lease exit expense in interest | 0.1 | 
| Component of lease exit payments in selling, general and administrative expenses | 0.4 | 
| Component of lease exit payments in income (loss) from operations and the sale of dealerships before tax | $ 2.8 | 
| X | ||||||||||
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- Definition Lease exit expense interest. No definition available. 
 | 
| X | ||||||||||
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- Definition Lease exit income (expense) discontinued operations. No definition available. 
 | 
| X | ||||||||||
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- Definition Lease exit payment discontinued operations. No definition available. 
 | 
| X | ||||||||||
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- Definition Lease exit payment selling general and administrative. No definition available. 
 | 
| X | ||||||||||
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- References No definition available. 
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