Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Additional Information (Detail)

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Summary of Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Significant Accounting Policies [Line Items]          
Effective tax rate from continuing operations 40.00% 38.00% 39.50% 38.70%  
Minimum [Member]          
Significant Accounting Policies [Line Items]          
Expected effective tax rate for continuing operations Range     38.00%    
Maximum [Member]          
Significant Accounting Policies [Line Items]          
Expected effective tax rate for continuing operations Range     40.00%    
Dealership [Member]          
Significant Accounting Policies [Line Items]          
Percentage of dealership that is accounted for under the equity method 50.00%   50.00%   50.00%