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Long-Term Debt (Restrictions And Covenants) (Details) (Credit Facility Due 2019 [Member], USD $)
Mar. 31, 2015
Credit Facility Due 2019 [Member]
 
Debt Instrument [Line Items]  
Capitalization ratio, requirement 70.00%an_DebtInstrumentCovenantTermsMaximumCapitalizationRatio
/ us-gaap_DebtInstrumentAxis
= an_CreditFacilityDue2019Member
Leverage ratio, requirement 3.75an_DebtInstrumentCovenantTermsMaximumLeverageRatio
/ us-gaap_DebtInstrumentAxis
= an_CreditFacilityDue2019Member
The maximum threshold of letters of credit excluded from the leverage ratio calculation $ 150,000,000an_DebtCovenantLettersOfCreditExcludedFromDebtLeverageRatioCovenant
/ us-gaap_DebtInstrumentAxis
= an_CreditFacilityDue2019Member
Capitalization ratio calculation, additions to shareholders' equity $ 1,530,000,000an_DebtCovenantCapitalizationRatioCalculationAdditionsToShareholdersEquity
/ us-gaap_DebtInstrumentAxis
= an_CreditFacilityDue2019Member