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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
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Inventory [Line Items] |  |
Inventory | $ 2,928.4 us-gaap_InventoryNet | $ 2,899.0 us-gaap_InventoryNet
New Vehicle [Member] |  |
Inventory [Line Items] |  |
Inventory | 2,291.8 us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_NewVehicleMember | 2,294.3 us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_NewVehicleMember
Used Vehicle [Member] |  |
Inventory [Line Items] |  |
Inventory | 470.6 us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_UsedVehicleMember | 437.6 us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_UsedVehicleMember
Parts and Service [Member] |  |
Inventory [Line Items] |  |
Inventory | $ 166.0 us-gaap_InventoryNet / us-gaap_PublicUtilitiesInventoryAxis = an_PartsAndServiceMember | $ 167.1 us-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 + Details
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