Segment Information (Details) $ in Millions		3 Months Ended
		Mar. 31, 2020 USD ($) segments	Mar. 31, 2019 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments		3	3
Revenues		$ 4,667.0	$ 4,981.8
Segment income		200.2	213.1
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		1,483.5	1,568.8
Segment income	[1]	54.1	56.2
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		1,362.1	1,496.1
Segment income	[1]	65.9	72.6
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		1,616.8	1,734.1
Segment income	[1]	$ 80.2	$ 84.3

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