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Inventory And Vehicle Floorplan Payable (Components Of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
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Floorplan Payable [Line Items] |  |
Vehicle floorplan payable | $ 4,031.1 | $ 3,997.7
Trade [Member] |  |
Floorplan Payable [Line Items] |  |
Vehicle floorplan payable | 2,453.7 | 2,388.0
Non-Trade [Member] |  |
Floorplan Payable [Line Items] |  |
Vehicle floorplan payable | $ 1,577.4 | $ 1,609.7

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- Definition Floorplan Payable [Line Items] + References No definition available. + Details
Name: | an_FloorplanPayableLineItems
Namespace Prefix: | an_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amounts borrowed to finance the purchase of specific new vehicle inventories with the corresponding manufacturers' captive finance subsidiaries ("trade lenders") and specific new and, to a lesser extent, used vehicle inventories with non-trade lenders (lenders that are not captive finance subsidiaries of the manufacturer). + References No definition available. + Details
Name: | an_VehicleFloorplanPayable
Namespace Prefix: | an_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | an_FloorplanPayableAxis=an_TradeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | an_FloorplanPayableAxis=an_NonTradeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |