Segment Information - Revenues and Segment Income by Reportable Segment (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) franchise	Jun. 30, 2024 USD ($)
Segment Reporting Information
Revenues	$ 7,662.3	$ 7,696.7	$ 15,266.8	$ 15,144.5
Cost of sales	6,365.7	6,432.6	12,701.2	12,635.2
Selling, general, and administrative expenses:
Personnel expenses	551.4	528.3	1,082.9	1,049.4
Advertising expenses	32.5	34.3	62.5	67.5
Rent & related expenses	108.2	103.7	213.5	207.1
Other expenses	214.2	221.2	461.0	443.3
Depreciation	42.1	38.6	82.0	76.4
Floor plan interest expense	43.3	46.6	84.8	91.4
Other interest expense	21.6	19.9	44.1	41.2
Equity in earnings of affiliates	53.6	54.0	86.9	87.3
Segment income	336.9	325.5	674.0	620.3
Gain on sale of dealership	0.0	0.0	52.3	0.0
Retail Automotive
Segment Reporting Information
Revenues	6,517.5	6,615.4	13,086.8	13,093.4
Cost of sales	5,408.7	5,540.4	10,894.2	10,961.2
Selling, general, and administrative expenses:
Personnel expenses	466.8	444.8	913.5	882.2
Advertising expenses	30.9	33.1	59.6	65.1
Rent & related expenses	97.9	94.1	193.1	187.4
Other expenses	201.0	202.0	426.6	406.0
Depreciation	36.5	33.5	71.0	66.6
Floor plan interest expense	36.2	39.5	71.7	78.3
Other interest expense	17.5	18.8	35.5	40.0
Equity in earnings of affiliates	0.3	1.2	0.5	2.2
Segment income	222.3	210.4	474.4	408.8
Retail Automotive | U.S.
Segment Reporting Information
Revenues	3,795.1	3,712.7	$ 7,486.1	7,127.7
Selling, general, and administrative expenses:
Number of retail automotive franchises sold | franchise			1
Retail Commercial Truck
Segment Reporting Information
Revenues	943.6	892.3	$ 1,767.3	1,684.1
Cost of sales	800.0	747.8	1,482.7	1,394.8
Selling, general, and administrative expenses:
Personnel expenses	65.4	64.8	131.3	131.0
Advertising expenses	0.8	0.7	1.7	1.5
Rent & related expenses	6.8	6.2	13.4	12.7
Other expenses	7.4	12.7	22.2	26.0
Depreciation	4.1	3.7	8.1	7.2
Floor plan interest expense	4.9	4.7	8.6	8.7
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	54.2	51.7	99.3	102.2
Other
Segment Reporting Information
Revenues	201.2	189.0	412.7	367.0
Cost of sales	157.0	144.4	324.3	279.2
Selling, general, and administrative expenses:
Personnel expenses	19.2	18.7	38.1	36.2
Advertising expenses	0.8	0.5	1.2	0.9
Rent & related expenses	3.5	3.4	7.0	7.0
Other expenses	5.8	6.5	12.2	11.3
Depreciation	1.5	1.4	2.9	2.6
Floor plan interest expense	2.2	2.4	4.5	4.4
Other interest expense	4.1	1.1	8.6	1.2
Equity in earnings of affiliates	0.0	0.0	0.0	0.0
Segment income	7.1	10.6	13.9	24.2
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	53.3	52.8	86.4	85.1
Segment income	$ 53.3	$ 52.8
Non-Automotive Investments
Segment Reporting Information
Revenues			0.0	0.0
Selling, general, and administrative expenses:
Segment income			$ 86.4	$ 85.1