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Inventory and Vehicle Floorplan Payable (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,097.2an_VehicleFloorplanPayable	3,029.0an_VehicleFloorplanPayable
Used vehicle floorplan facilities, current borrowing capacity	50.2an_UsedVehicleFloorplanFacilityCurrentBorrowingCapacity
Used vehicle floorplan facilities, remaining borrowing capacity	79.0an_UsedVehicleFloorplanFacilityRemainingBorrowingCapacity
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	236.0an_VehicleFloorplanPayable / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_UsedVehicleFloorplanFacilitiesMember
Vehicle floorplan facilities, maximum borrowing capacity	315.0an_VehicleFloorplanFacilityMaximumBorrowingCapacity / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_UsedVehicleFloorplanFacilitiesMember
Vehicle floorplan facilities, average LIBOR-based interest rates	1.70%an_VehicleFloorplanFacilityInterestRateDuringPeriod / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_UsedVehicleFloorplanFacilitiesMember	1.80%an_VehicleFloorplanFacilityInterestRateDuringPeriod / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_UsedVehicleFloorplanFacilitiesMember
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	2,900.0an_VehicleFloorplanPayable / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_NewVehicleFloorplanFacilitiesMember
Vehicle floorplan facilities, maximum borrowing capacity	$ 3,600.0an_VehicleFloorplanFacilityMaximumBorrowingCapacity / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_NewVehicleFloorplanFacilitiesMember
Vehicle floorplan facilities, average LIBOR-based interest rates	1.80%an_VehicleFloorplanFacilityInterestRateDuringPeriod / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_NewVehicleFloorplanFacilitiesMember	1.90%an_VehicleFloorplanFacilityInterestRateDuringPeriod / an_FloorplanFacilitiesNewVehicleUsedVehicleAxis = an_NewVehicleFloorplanFacilitiesMember

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- Details

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- Definition

Amount of current borrowing capacity under used vehicle floorplan facilities considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of eligible used vehicle inventory that could have been pledged as collateral) and any amounts currently outstanding under the facility.

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No definition available.

+ Details

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Namespace Prefix:	an_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Amount of borrowing capacity currently available under used vehicle floorplan facilities (maximum borrowing capacity less the amount of borrowings outstanding).

+ References

No definition available.

+ Details

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Namespace Prefix:	an_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

The average effective interest rate of vehicle floorplan facilities during the reporting period.

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No definition available.

+ Details

Name:	an_VehicleFloorplanFacilityInterestRateDuringPeriod
Namespace Prefix:	an_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

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- Definition

The aggregate maximum borrowing capacity under new and used vehicle floorplan facilities without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facilities.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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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

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- Details

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- Details

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Period Type: