Segment Information (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segments	Sep. 30, 2018 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments				3	3
Revenues		$ 5,461.2	$ 5,349.2	$ 15,786.8	$ 16,001.1
Segment income		246.2	229.4	701.9	680.3
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		1,728.6	1,789.7	5,004.7	5,389.5
Segment income	[1]	70.3	67.2	192.4	194.6
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		1,722.5	1,770.6	4,845.9	5,140.2
Segment income	[1]	86.9	85.3	240.9	236.2
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		1,820.5	1,668.8	5,375.5	5,116.8
Segment income	[1]	$ 89.0	$ 76.9	$ 268.6	$ 249.5

[1] Segment income represents income for each of our reportable segments and is defined as operating income less floorplan interest expense.