Inventory and Vehicle Floorplan Payable (Components of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 2,759.9	$ 3,575.8
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	1,541.7	2,120.6
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,218.2	$ 1,455.2

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).

No definition available.

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