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Summary of Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Significant Accounting Policies [Line Items]    
Percentage of dealership that is accounted for under the equity method 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage  
Effective tax rate from continuing operations 39.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 38.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Minimum [Member]    
Significant Accounting Policies [Line Items]    
Expected effective tax rate for continuing operations Range 38.00%sah_EffectiveIncomeTaxRateContinuingOperationsFuturePeriod
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		  
Maximum [Member]    
Significant Accounting Policies [Line Items]    
Expected effective tax rate for continuing operations Range 40.00%sah_EffectiveIncomeTaxRateContinuingOperationsFuturePeriod
/ us-gaap_RangeAxis
= us-gaap_MaximumMember