Segment Information (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segments	Jun. 30, 2019 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments				3	3
Revenues		$ 4,533.0	$ 5,343.8	$ 9,200.0	$ 10,325.6
Segment income		259.6	242.6	459.8	455.7
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		1,486.0	1,707.3	2,969.5	3,276.1
Segment income	[1]	82.1	65.9	136.2	122.1
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		1,325.3	1,627.3	2,687.4	3,123.4
Segment income	[1]	88.3	81.4	154.2	154.0
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		1,564.8	1,820.9	3,181.6	3,555.0
Segment income	[1]	$ 89.2	$ 95.3	$ 169.4	$ 179.6

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