Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies - Additional Information (Detail)

v2.4.1.9
Commitments and Contingencies - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Option
Dec. 31, 2013
Dec. 31, 2012
Contingencies And Commitments [Line Items]      
Lease exit expense $ 0.3us-gaap_BusinessExitCosts1    
Interest Expense 2.0us-gaap_InterestExpense    
Lease exit accrued cost 1.7sah_LeaseExitAccrualsAmount    
Operating lease term for dealership facilities, minimum 15 years    
Operating lease term for dealership facilities, maximum 20 years    
Operating lease number of renewal options, minimum 1sah_NumberOfOperatingLeaseRenewalOptionsMinimum    
Operating lease number of renewal options, maximum 2sah_OperatingLeaseNumberOfRenewalOptionsMaximum    
Operating lease period of renewal options 10 years    
Percentage of lease facility based on capitalization rates 20.00%sah_PercentageLeaseFacilityBasedOnCapitalizationRates    
Lease expense for continuing operation 106.0sah_LeaseExpenseForContinuingOperation 99.6sah_LeaseExpenseForContinuingOperation 102.4sah_LeaseExpenseForContinuingOperation
Lease expense for discontinuing operation 0.9sah_LeaseExpenseForDiscontinuingOperation 1.1sah_LeaseExpenseForDiscontinuingOperation 6.2sah_LeaseExpenseForDiscontinuingOperation
Leases, net contingent rent benefit related to decrease in interest rates from continuing operation 2.0sah_LeasesContingentRentBenefitContinuingOperation 2.3sah_LeasesContingentRentBenefitContinuingOperation 2.3sah_LeasesContingentRentBenefitContinuingOperation
Leases net contingent rent benefit related to decrease in interest rates from discontinuing operation 0.1sah_LeasesContingentRentBenefitDiscontinuingOperation 0.6sah_LeasesContingentRentBenefitDiscontinuingOperation 0.6sah_LeasesContingentRentBenefitDiscontinuingOperation
Obligations under subleases, if subleases do not perform 91.1sah_ObligationsUnderSubleasesAmount    
Estimated minimum lease payment 5.2sah_LeaseObligationsIfBuyerDoesNotPerformAmount    
Maximum exposure associated with general indemnifications 16.8us-gaap_AccrualForEnvironmentalLossContingenciesGross    
General indemnifications minimum expiration period 1 year    
General indemnifications maximum expiration period 3 years    
Joint venture ownership percentage 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage    
Contingent liability reserve balance after reduction 2.8sah_ContingentLiabilityReserveBalance    
Other Accrued Liabilities [Member]      
Contingencies And Commitments [Line Items]      
Amount reserved for pending proceedings 2.0sah_LegalMattersAmountReservedForPendingProceedings
/ us-gaap_BalanceSheetLocationAxis
= sah_OtherAccruedLiabilitiesMember 		1.2sah_LegalMattersAmountReservedForPendingProceedings
/ us-gaap_BalanceSheetLocationAxis
= sah_OtherAccruedLiabilitiesMember 		 
Other Long-Term Liabilities [Member]      
Contingencies And Commitments [Line Items]      
Amount reserved for pending proceedings $ 0.3sah_LegalMattersAmountReservedForPendingProceedings
/ us-gaap_BalanceSheetLocationAxis
= sah_OtherLongTermLiabilitiesMember