Segments (Schedule of Certain Financial Information on a Segment Basis) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total revenues	$ 3,620.2	$ 3,332.4	$ 9,182.5	$ 9,403.8
Income before income taxes	219.1	117.4	391.6	280.7
Depreciation and amortization			(67.3)	(60.9)
Other interest expense	(16.6)	(14.8)	(50.4)	(45.0)
Income before income taxes	2.2	3.3	8.2	8.9
Operating segments
Segment Reporting Information [Line Items]
Total revenues	3,616.1	3,324.9	9,169.9	9,377.1
Income before income taxes	222.9	99.2	397.6	248.4
Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	1,214.7	1,157.8	3,286.9	3,265.8
Income before income taxes	81.1	37.9	168.3	94.1
Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	1,550.5	1,437.7	3,832.3	3,962.3
Income before income taxes	102.6	48.3	176.2	117.8
Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	850.9	729.4	2,050.7	2,149.0
Income before income taxes	39.2	13.0	53.1	36.5
Corporate and other
Segment Reporting Information [Line Items]
Total revenues	4.1	7.5	12.6	26.7
Income before income taxes	33.5	50.6	103.5	129.3
Segment reconciling items
Segment Reporting Information [Line Items]
Depreciation and amortization	(22.9)	(20.9)	(67.3)	(60.9)
Other interest expense	(16.6)	(14.8)	(50.4)	(45.0)
Income before income taxes	2.2	3.3	8.2	8.9
New vehicle retail
Segment Reporting Information [Line Items]
Total revenues	1,883.3	1,824.8	4,624.6	4,993.3
New vehicle retail | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	621.3	613.0	1,591.0	1,682.7
New vehicle retail | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	822.9	824.8	1,995.9	2,198.1
New vehicle retail | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	440.8	387.0	1,039.0	1,110.2
Used vehicle retail
Segment Reporting Information [Line Items]
Total revenues	1,093.2	916.3	2,889.7	2,632.4
Used vehicle retail | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	378.8	328.1	1,096.4	949.6
Used vehicle retail | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	451.9	380.9	1,152.6	1,085.7
Used vehicle retail | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	261.8	206.7	642.5	596.0
Used vehicle wholesale
Segment Reporting Information [Line Items]
Total revenues	98.8	74.4	216.8	233.5
Used vehicle wholesale | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	31.9	30.3	77.7	89.5
Used vehicle wholesale | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	42.2	28.4	88.6	85.3
Used vehicle wholesale | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	24.8	16.4	50.3	59.0
Finance and insurance
Segment Reporting Information [Line Items]
Total revenues	160.5	136.3	407.2	382.7
Finance and insurance | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	53.2	48.1	149.2	137.2
Finance and insurance | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	81.1	65.2	195.0	180.9
Finance and insurance | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	25.6	19.2	60.2	54.1
Service, body and parts
Segment Reporting Information [Line Items]
Total revenues	359.5	340.5	964.9	993.3
Service, body and parts | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	118.0	122.8	337.5	359.2
Service, body and parts | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	141.3	129.2	371.3	374.2
Service, body and parts | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	96.0	84.9	244.8	248.0
Fleet and other
Segment Reporting Information [Line Items]
Total revenues	24.9	40.1	79.3	168.6
Fleet and other | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	11.5	15.5	35.1	47.6
Fleet and other | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	11.1	9.2	28.9	38.1
Fleet and other | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	$ 1.9	$ 15.2	$ 13.9	$ 81.7