Segment Information (Revenues and Segment Income) (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2022 USD ($) segments	Sep. 30, 2021 USD ($) segments
Segment Reporting [Abstract]
Number of reportable segments | segments				3	3
Segment Reporting Information [Line Items]
Revenues		$ 6,666.0	$ 6,379.5	$ 20,288.0	$ 19,261.7
Segment income		558.2	555.9	1,726.4	1,557.2
Domestic
Segment Reporting Information [Line Items]
Revenues		2,032.8	1,955.2	6,108.1	5,926.7
Segment income	[1]	142.7	149.1	445.2	436.6
Import
Segment Reporting Information [Line Items]
Revenues		1,875.2	1,983.3	5,799.0	5,927.9
Segment income	[1]	180.3	200.7	559.0	530.3
Premium Luxury
Segment Reporting Information [Line Items]
Revenues		2,506.4	2,218.0	7,601.7	6,790.0
Segment income	[1]	$ 235.2	$ 206.1	$ 722.2	$ 590.3

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