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Inventory And Vehicle Floorplan Payable (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Floorplan Payable [Line Items]
Vehicle floorplan facilities, amount outstanding	$ 3,603.3		$ 3,806.9
Used vehicle floorplan facilities, remaining borrowing capacity	133.0
Used vehicle floorplan facilities, current borrowing capacity	$ 0.5
New Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.40%	2.70%
Vehicle floorplan facilities, maximum borrowing capacity	$ 4,800.0
Vehicle floorplan facilities, amount outstanding	$ 3,200.0
Used Vehicle Floorplan Facilities [Member]
Floorplan Payable [Line Items]
Vehicle floorplan facilities, average LIBOR-based interest rates (percent)	3.40%	2.70%
Vehicle floorplan facilities, maximum borrowing capacity	$ 500.0
Vehicle floorplan facilities, amount outstanding	$ 367.0

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- Definition

Floorplan Payable [Line Items]

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

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

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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.

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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).

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