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Goodwill and Intangible Assets (Goodwill) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Goodwill [Roll Forward]
Goodwill, beginning balance	1,259,600,000us-gaap_Goodwill	[1]	1,237,400,000us-gaap_Goodwill	[1]
Acquisitions and other adjustments	55,100,000us-gaap_GoodwillPeriodIncreaseDecrease		22,200,000us-gaap_GoodwillPeriodIncreaseDecrease
Goodwill, ending balance	1,314,700,000us-gaap_Goodwill	[1]	1,259,600,000us-gaap_Goodwill	[1]
Accumulated impairment losses	1,470,000,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss		1,470,000,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss		1,470,000,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss
Accumulated impairment losses, after-tax	1,250,000,000an_GoodwillImpairedAccumulatedImpairmentLossNetOfTax		1,250,000,000an_GoodwillImpairedAccumulatedImpairmentLossNetOfTax		1,250,000,000an_GoodwillImpairedAccumulatedImpairmentLossNetOfTax
Goodwill	1,314,700,000us-gaap_Goodwill	[1]	1,259,600,000us-gaap_Goodwill	[1]
Goodwill [Member]
Goodwill [Line Items]
Date of annual goodwill and indefinite lived intangible assets impairment test	April 30		April 30		April 30
Nonrecurring [Member]
Goodwill [Line Items]
Goodwill impairment	0us-gaap_GoodwillImpairmentLoss / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember		0us-gaap_GoodwillImpairmentLoss / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember		0us-gaap_GoodwillImpairmentLoss / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Reporting Unit, Domestic [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	165,200,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember	[1]	165,200,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember	[1]
Acquisitions and other adjustments	9,900,000us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember		0us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember
Goodwill, ending balance	175,100,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember	[1]	165,200,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember	[1]
Accumulated impairment losses	140,000,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember		140,000,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember		140,000,000us-gaap_GoodwillImpairedAccumulatedImpairmentLoss / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember
Accumulated impairment losses, after-tax	119,000,000an_GoodwillImpairedAccumulatedImpairmentLossNetOfTax / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember		119,000,000an_GoodwillImpairedAccumulatedImpairmentLossNetOfTax / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember		119,000,000an_GoodwillImpairedAccumulatedImpairmentLossNetOfTax / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember
Goodwill	175,100,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember	[1]	165,200,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitDomesticMember	[1]
Reporting Unit, Import [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	555,800,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember	[1]	534,200,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember	[1]
Acquisitions and other adjustments	(4,200,000)us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember		21,600,000us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember
Goodwill, ending balance	551,600,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember	[1]	555,800,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember	[1]
Goodwill	551,600,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember	[1]	555,800,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitImportMember	[1]
Reporting Unit, Premium Luxury [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	538,600,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember	[1]	538,000,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember	[1]
Acquisitions and other adjustments	49,400,000us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember		600,000us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember
Goodwill, ending balance	588,000,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember	[1]	538,600,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember	[1]
Goodwill	588,000,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember	[1]	538,600,000us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_ReportingUnitPremiumLuxuryMember	[1]
Goodwill Corporate and Other [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	0us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember	[1]	0us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember	[1]
Acquisitions and other adjustments	0us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember		0us-gaap_GoodwillPeriodIncreaseDecrease / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember
Goodwill, ending balance	0us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember	[1]	0us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember	[1]
Goodwill	0us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember	[1]	0us-gaap_Goodwill / an_GoodwillReportingUnitsAxis = an_GoodwillCorporateAndOtherMember	[1]
[1] Net of accumulated impairment losses of $1.47 billion ($1.25 billion after-tax) associated with our single reporting unit (prior to September 30, 2008, our reporting unit structure was comprised of a single reporting unit) and $140.0 million ($119.0 million after-tax) associated with our Domestic reporting unit, both of which were recorded during the year ended December 31, 2008.

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