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Inventory And Vehicle Floorplan Payable (Components Of Inventory) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Inventory [Line Items]
Inventory	$ 2,928.4us-gaap_InventoryNet	$ 2,899.0us-gaap_InventoryNet
New Vehicle [Member]
Inventory [Line Items]
Inventory	2,291.8us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_NewVehicleMember	2,294.3us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_NewVehicleMember
Used Vehicle [Member]
Inventory [Line Items]
Inventory	470.6us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_UsedVehicleMember	437.6us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_UsedVehicleMember
Parts and Service [Member]
Inventory [Line Items]
Inventory	$ 166.0us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_PartsAndServiceMember	$ 167.1us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_PartsAndServiceMember

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- Definition

Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.6(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 330

-SubTopic 10

-Section 35

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6386567&loc=d3e3927-108312



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765

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