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Stock-Based Compensation (Restricted Stock Units) (Details) (Restricted Stock Units (RSUs) [Member], USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
May 06, 2014
Jan. 02, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Weighted-average grant date fair value of RSU's granted (in dollars per share) $ 53.57us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue $ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue $ 0.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue    
Total fair value of RSU's granted $ 2.1us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue $ 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue    
Director [Member] | Non-Employee Director Equity Plan 2014 [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of restricted stock units granted to each non-employee director       5,000an_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofRestrictedStockUnitsGrantedtoEachGrantee
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= an_NonEmployeeDirectorEquityPlan2014Member
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember
 
Subsequent Event [Member] | Director [Member] | Non-Employee Director Equity Plan 2014 [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of restricted stock units granted to each non-employee director         5,000an_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofRestrictedStockUnitsGrantedtoEachGrantee
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_PlanNameAxis
= an_NonEmployeeDirectorEquityPlan2014Member
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember