Segment Information - Revenues and Segment Income by Reportable Segment (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) franchise	Sep. 30, 2024 USD ($)
Segment Reporting Information
Revenues	$ 7,695.3	$ 7,590.8	$ 22,962.1	$ 22,735.3
Cost of sales	6,446.2	6,347.6	19,147.4	18,982.8
Selling, general, and administrative expenses:
Personnel expenses	538.6	530.0	1,621.5	1,579.4
Advertising expenses	33.5	33.9	96.1	101.4
Rent & related expenses	109.6	106.6	323.0	313.7
Other expenses	225.8	214.7	686.8	658.0
Depreciation	42.9	40.6	124.9	117.0
Floor plan interest expense	42.9	50.8	127.7	142.2
Other interest expense	22.0	22.9	66.1	64.1
Equity in earnings of affiliates	58.4	60.7	145.3	148.0
Segment income	292.2	304.4	966.2	924.7
Gain on sale of dealership	0.0	0.0	52.3	0.0
Retail Automotive
Segment Reporting Information
Revenues	6,570.1	6,340.7	19,656.9	19,434.1
Cost of sales	5,503.8	5,299.2	16,398.0	16,260.4
Selling, general, and administrative expenses:
Personnel expenses	456.3	442.9	1,369.8	1,324.9
Advertising expenses	32.3	32.4	92.0	97.6
Rent & related expenses	99.0	96.5	292.0	283.9
Other expenses	205.8	193.6	632.4	600.0
Depreciation	37.2	35.3	108.2	101.7
Floor plan interest expense	36.1	41.8	107.8	120.0
Other interest expense	18.8	21.8	54.3	61.8
Equity in earnings of affiliates	0.0	0.5	0.5	2.7
Segment income	180.8	177.7	655.2	586.5
Retail Automotive | U.S.
Segment Reporting Information
Revenues	3,863.9	3,584.3	$ 11,350.0	10,712.0
Selling, general, and administrative expenses:
Number of retail automotive franchises sold | franchise			2
Retail Commercial Truck
Segment Reporting Information
Revenues	918.6	1,063.3	$ 2,685.9	2,747.4
Cost of sales	782.3	906.2	2,265.0	2,301.0
Selling, general, and administrative expenses:
Personnel expenses	63.7	68.6	195.0	199.7
Advertising expenses	0.9	0.9	2.6	2.4
Rent & related expenses	6.6	6.6	20.0	19.3
Other expenses	14.5	14.3	36.7	40.1
Depreciation	4.1	3.9	12.2	11.2
Floor plan interest expense	5.0	6.3	13.6	15.0
Other interest expense	0.0	0.0	0.0	0.0
Equity in earnings of affiliates	0.0	0.0	0.0	0.0
Segment income	41.5	56.5	140.8	158.7
Other
Segment Reporting Information
Revenues	206.6	186.8	619.3	553.8
Cost of sales	160.1	142.2	484.4	421.4
Selling, general, and administrative expenses:
Personnel expenses	18.6	18.5	56.7	54.8
Advertising expenses	0.3	0.6	1.5	1.4
Rent & related expenses	4.0	3.5	11.0	10.5
Other expenses	5.5	6.8	17.7	17.9
Depreciation	1.6	1.4	4.5	4.1
Floor plan interest expense	1.8	2.7	6.3	7.2
Other interest expense	3.2	1.1	11.8	2.3
Equity in earnings of affiliates	0.0	0.0	0.0	0.0
Segment income	11.5	10.0	25.4	34.2
Non-Automotive Investments
Segment Reporting Information
Revenues	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Selling, general, and administrative expenses:
Personnel expenses	0.0	0.0	0.0	0.0
Advertising expenses	0.0	0.0	0.0	0.0
Rent & related expenses	0.0	0.0	0.0	0.0
Other expenses	0.0	0.0	0.0	0.0
Depreciation	0.0	0.0	0.0	0.0
Floor plan interest expense	0.0	0.0	0.0	0.0
Other interest expense	0.0	0.0	0.0	0.0
Equity in earnings of affiliates	58.4	60.2	144.8	145.3
Segment income	$ 58.4	$ 60.2
Non-Automotive Investments
Segment Reporting Information
Revenues			0.0	0.0
Selling, general, and administrative expenses:
Segment income			$ 144.8	$ 145.3