Segments (Schedule of Certain Financial Information on a Segment Basis) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 3,332.4	$ 3,092.0	$ 9,403.8	$ 8,848.2
Income before income taxes	117.4	109.0	280.7	259.5
Depreciation and amortization			(60.9)	(55.3)
Other interest expense	(14.8)	(15.0)	(45.0)	(40.7)
Other income, net	3.3	2.4	8.9	5.4
Operating segments
Segment Reporting Information [Line Items]
Total revenues	3,324.9	3,087.1	9,377.1	8,840.0
Income before income taxes	99.2	74.4	248.4	200.6
Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	1,157.8	1,097.8	3,265.8	3,163.6
Income before income taxes	37.9	25.3	94.1	79.5
Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	1,437.7	1,336.4	3,962.3	3,829.5
Income before income taxes	48.3	37.4	117.8	90.6
Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	729.4	652.9	2,149.0	1,846.9
Income before income taxes	13.0	11.7	36.5	30.5
Corporate and other
Segment Reporting Information [Line Items]
Total revenues	7.5	4.9	26.7	8.2
Income before income taxes	50.6	66.8	129.3	149.5
Segment reconciling items
Segment Reporting Information [Line Items]
Depreciation and amortization	(20.9)	(19.6)	(60.9)	(55.3)
Other interest expense	(14.8)	(15.0)	(45.0)	(40.7)
Other income, net	3.3	2.4	8.9	5.4
New vehicle
Segment Reporting Information [Line Items]
Total revenues	1,824.8	1,733.0	4,993.3	4,914.5
New vehicle | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	613.0	600.1	1,682.7	1,711.2
New vehicle | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	824.8	787.8	2,198.1	2,227.4
New vehicle | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	387.0	347.6	1,110.2	988.0
Used vehicle retail
Segment Reporting Information [Line Items]
Total revenues	916.3	805.9	2,632.4	2,325.6
Used vehicle retail | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	328.1	288.2	949.6	836.5
Used vehicle retail | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	380.9	334.6	1,085.7	977.9
Used vehicle retail | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	206.7	182.5	596.0	510.3
Used vehicle wholesale
Segment Reporting Information [Line Items]
Total revenues	74.4	92.0	233.5	253.2
Used vehicle wholesale | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	30.3	37.0	89.5	104.5
Used vehicle wholesale | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	28.4	35.2	85.3	94.3
Used vehicle wholesale | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	16.4	19.9	59.0	54.4
Finance and insurance
Segment Reporting Information [Line Items]
Total revenues	136.3	121.1	382.7	342.1
Finance and insurance | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	48.1	42.5	137.2	125.6
Finance and insurance | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	65.2	60.5	180.9	167.1
Finance and insurance | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	19.2	16.4	54.1	44.6
Service, body and parts
Segment Reporting Information [Line Items]
Total revenues	340.5	311.3	993.3	908.4
Service, body and parts | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	122.8	114.7	359.2	335.4
Service, body and parts | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	129.2	114.8	374.2	340.8
Service, body and parts | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	84.9	76.9	248.0	218.6
Fleet and other
Segment Reporting Information [Line Items]
Total revenues	40.1	28.7	168.6	104.4
Fleet and other | Operating segments | Domestic
Segment Reporting Information [Line Items]
Total revenues	15.5	15.3	47.6	50.4
Fleet and other | Operating segments | Import
Segment Reporting Information [Line Items]
Total revenues	9.2	3.5	38.1	22.0
Fleet and other | Operating segments | Luxury
Segment Reporting Information [Line Items]
Total revenues	$ 15.2	$ 9.6	$ 81.7	$ 31.0