Commitments and Contingencies - Summary of Lease Exit Accruals (Parenthetical) (Detail) $ in Millions  | 
12 Months Ended | 
|---|---|
| 
 Dec. 31, 2016  
USD ($) 
 | |
| Commitments And Contingencies Disclosure [Abstract] | |
| Component of lease exit expense in interest expense, other, net | $ 0.1 | 
| Component of lease exit expense in selling, general and administrative expenses | 0.3 | 
| Component of lease exit expense in income (loss) from operations and the sale of dealerships | 1.0 | 
| Component of lease exit payments in selling, general and administrative expenses | 0.7 | 
| Component of lease exit payments in income (loss) from operations and the sale of dealerships | $ 5.4 | 
| X | ||||||||||
- Definition Lease exit expense interest. No definition available. 
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- Definition Lease exit expense selling general and administrative. No definition available. 
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| X | ||||||||||
- Definition Lease exit income (expense) discontinued operations. No definition available. 
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| X | ||||||||||
- Definition Lease exit payment discontinued operations. No definition available. 
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| X | ||||||||||
- Definition Lease exit payment selling general and administrative. No definition available. 
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| X | ||||||||||
- References No definition available. 
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