Segment Information - Revenues and Segment Income by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information
Revenues	$ 7,590.8	$ 7,447.8	$ 22,735.3	$ 22,255.3
Segment income	304.4	356.5	924.7	1,163.7
Retail automotive dealership
Segment Reporting Information
Revenues	6,340.7	6,325.4	19,434.1	19,031.2
Retail automotive dealership | Operating segments
Segment Reporting Information
Revenues	6,340.7	6,325.4	19,434.1	19,031.2
Segment income	177.7	200.8	586.5	718.6
Retail commercial truck dealership
Segment Reporting Information
Revenues	1,063.3	964.7	2,747.4	2,779.5
Retail commercial truck dealership | Operating segments
Segment Reporting Information
Revenues	1,063.3	964.7	2,747.4	2,779.5
Segment income	56.5	61.1	158.7	173.7
Other | Operating segments
Segment Reporting Information
Revenues	186.8	157.7	553.8	444.6
Segment income	10.0	10.4	34.2	32.9
Non-Automotive Investments | Operating segments
Segment Reporting Information
Revenues	0.0	0.0	0.0	0.0
Segment income	$ 60.2	$ 84.2	$ 145.3	$ 238.5