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Description of Business and Summary of Significant Accounting Policies (Employee Savings Plans) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
401(k) plan employer matching contribution $ 5.9us-gaap_DefinedContributionPlanCostRecognized $ 5.2us-gaap_DefinedContributionPlanCostRecognized $ 3.5us-gaap_DefinedContributionPlanCostRecognized
Balances due to participants 63.4us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent 51.7us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent  
Employer Matching Contribution [Member]      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Matching contributions to employee participants $ 0.6us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= an_EmployerMatchingContributionMember
$ 0.5us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= an_EmployerMatchingContributionMember
$ 0.5us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= an_EmployerMatchingContributionMember
Deferred compensation arrangement with individual matching contribution annual vesting years two and three 33.00%an_DeferredCompensationArrangementWithIndividualMatchingContributionAnnualVestingYearsTwoAndThree
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= an_EmployerMatchingContributionMember
   
Deferred compensation arrangement with individual matching contribution annual vesting portion, year one 33.00%an_DeferredCompensationArrangementWithIndividualMatchingContributionAnnualVestingPortionYearOne
/ us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis
= an_EmployerMatchingContributionMember
   
Employer discretionary contribution [Member]      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Employer contributions vesting periods (in years) 3 years    
Employees Hired Subsequent to January 1, 2011 [Member]      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
401(k) plan matching contribution graded vesting period 3 years