Segment Information - Revenues and Segment Income by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting
Revenues	$ 8,415.6	$ 8,053.0	$ 24,791.9	$ 24,039.3
Cost of sales	7,091.5	6,753.4	20,811.4	20,066.1
Selling, general, and administrative expenses:
Personnel expenses	579.2	563.9	1,730.2	1,698.4
Advertising expenses	30.9	34.6	93.8	99.1
Rent & related expenses	113.7	113.2	344.6	334.1
Other expenses	251.7	233.1	746.5	708.4
Depreciation	45.5	43.8	135.7	127.5
Floor plan interest expense	38.5	43.4	115.1	129.2
Other interest expense	30.9	22.0	92.4	66.1
Equity in earnings of affiliates	68.3	58.4	166.4	145.3
Segment income	302.0	304.0	979.5	1,008.0
Gain on sale of dealerships	0.0	0.0	90.9	52.3
Retail Automotive
Segment Reporting
Revenues	7,025.4	6,927.8	21,293.5	20,734.1
Cost of sales	5,922.7	5,811.0	17,909.3	17,316.7
Selling, general, and administrative expenses:
Personnel expenses	488.2	481.6	1,469.5	1,446.7
Advertising expenses	29.4	33.4	89.8	95.0
Rent & related expenses	102.2	102.6	310.8	303.1
Other expenses	227.0	213.1	676.7	654.0
Depreciation	39.2	38.1	117.5	110.8
Floor plan interest expense	34.5	36.6	103.3	109.3
Other interest expense	28.5	18.8	85.0	54.3
Equity in earnings of affiliates	0.0	0.0	0.0	0.5
Segment income	153.7	192.6	622.5	697.0
Retail Commercial Truck
Segment Reporting
Revenues	1,123.9	918.6	2,746.3	2,685.9
Cost of sales	960.5	782.3	2,311.9	2,265.0
Selling, general, and administrative expenses:
Personnel expenses	71.1	63.7	200.3	195.0
Advertising expenses	1.0	0.9	2.4	2.6
Rent & related expenses	7.1	6.6	20.6	20.0
Other expenses	16.7	14.5	46.1	36.7
Depreciation	4.7	4.1	13.5	12.2
Floor plan interest expense	3.2	5.0	8.4	13.6
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	59.6	41.5	143.1	140.8
Other
Segment Reporting
Revenues	266.3	206.6	752.1	619.3
Cost of sales	208.3	160.1	590.2	484.4
Selling, general, and administrative expenses:
Personnel expenses	19.9	18.6	60.4	56.7
Advertising expenses	0.5	0.3	1.6	1.5
Rent & related expenses	4.4	4.0	13.2	11.0
Other expenses	8.0	5.5	23.7	17.7
Depreciation	1.6	1.6	4.7	4.5
Floor plan interest expense	0.8	1.8	3.4	6.3
Other interest expense	2.4	3.2	7.4	11.8
Equity in earnings of affiliates	0.0	0.0	0.0	0.0
Segment income	20.4	11.5	47.5	25.4
Non-Automotive Investments
Segment Reporting
Revenues	0.0	0.0	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	68.3	58.4	166.4	144.8
Segment income	$ 68.3	$ 58.4	$ 166.4	$ 144.8