Inventory And Vehicle Floorplan Payable (Components Of Vehicle Floorplan Payable) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 3,603.3	$ 3,806.9
Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	2,107.6	2,179.1
Non-Trade [Member]
Floorplan Payable [Line Items]
Vehicle floorplan payable	$ 1,495.7	$ 1,627.8

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