Annual report pursuant to Section 13 and 15(d)

Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail)

v2.4.1.9
Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)
 0 Months Ended 12 Months Ended
Oct. 02, 2014
Dec. 31, 2014
Vehicle
Franchise
Dealerships
Store
Collision
Brand
State
Dec. 31, 2013
Dec. 31, 2012
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Number of dealerships   118sah_NumberOfDealershipsOperatedByEntity    
Number of states   13us-gaap_NumberOfStatesInWhichEntityOperates    
Number of different brands of cars and light trucks   25sah_NumberOfBrandsOfCarsAndLightTrucks    
Number of collision repair centers   19sah_NumberOfCollisionRepairCenters    
Number of stores   100us-gaap_NumberOfStores    
Percentage of dealership that is accounted for under the equity method   50.00%sah_PercentageOfDealershipSubsidiariesAccountedAsEquityMethod    
Book overdraft position   $ 37,100,000us-gaap_BankOverdrafts $ 41,000,000us-gaap_BankOverdrafts  
Revenue allowances for commission reserves   15,400,000us-gaap_ValuationAllowancesAndReservesBalance 14,900,000us-gaap_ValuationAllowancesAndReservesBalance  
Amount recognized for continue operation   39,700,000sah_FloorPlanAssistanceRecognizedContinuingOperations 37,900,000sah_FloorPlanAssistanceRecognizedContinuingOperations 32,100,000sah_FloorPlanAssistanceRecognizedContinuingOperations
Additional amount recognized for discontinued operation       900,000sah_FloorPlanAssistanceRecognizedDiscontinuingOperations
Term for funding of finance contracts   10 days    
Contracts in transit receivables   194,000,000sah_ContractsInTransitReceivables 190,000,000sah_ContractsInTransitReceivables  
Goodwill impairment 0us-gaap_GoodwillImpairmentLoss 0us-gaap_GoodwillImpairmentLoss 0us-gaap_GoodwillImpairmentLoss 0us-gaap_GoodwillImpairmentLoss
Number of reporting units   2us-gaap_NumberOfReportingUnits    
Projected sales in units for next year   16,500,000sah_GoodwillValuationAssumptionProjectedSalesInUnit    
Goodwill   475,929,000us-gaap_Goodwill 476,315,000us-gaap_Goodwill 454,224,000us-gaap_Goodwill
Franchise asset impairment charge   2,200,000us-gaap_ImpairmentOfIntangibleAssetsIndefinitelivedExcludingGoodwill    
Insurance reserves   23,900,000us-gaap_SelfInsuranceReserve 23,600,000us-gaap_SelfInsuranceReserve  
Concentrations of credit risk with respect to receivables are limited primarily to automobile manufacturers   90,100,000sah_ConcentrationCreditRiskReceivables 82,600,000sah_ConcentrationCreditRiskReceivables  
Deposits   57,500,000us-gaap_DepositAssets 65,000,000us-gaap_DepositAssets  
Reduction in interest expense   2,100,000us-gaap_IncreaseDecreaseInInterestPayableNet 1,000,000us-gaap_IncreaseDecreaseInInterestPayableNet 300,000us-gaap_IncreaseDecreaseInInterestPayableNet
Total outstanding balance of financial instruments and market risks   1,400,000,000sah_FinancialInstrumentsOutstandingBalanceBeforeInterestRateSwaps 1,300,000,000sah_FinancialInstrumentsOutstandingBalanceBeforeInterestRateSwaps  
Advertising expense   57,400,000us-gaap_AdvertisingExpense 56,600,000us-gaap_AdvertisingExpense 50,300,000us-gaap_AdvertisingExpense
Cooperative manufacturer credits advertising expenses   23,400,000us-gaap_CooperativeAdvertisingAmount 21,800,000us-gaap_CooperativeAdvertisingAmount 22,000,000us-gaap_CooperativeAdvertisingAmount
Financial Institutions [Member]        
Description Of Business And Summary Of Significant Accounting Policies [Line Items]        
Concentrations of credit risk with respect to receivables are limited primarily to financial institutions   $ 215,400,000sah_ConcentrationCreditRiskAmount
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember 		$ 210,300,000sah_ConcentrationCreditRiskAmount
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_LenderConcentrationRiskMember