Segment Information - Revenues and Segment Income by Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2025	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 7,769.2	$ 8,053.0	$ 8,032.5	$ 7,953.8	$ 8,077.6	$ 7,929.5	$ 8,065.0	$ 7,792.7	$ 31,808.5	$ 31,864.8	$ 30,916.5
Total cost of sales									26,591.5	26,647.7	25,769.1
Selling, general, and administrative expenses:
Personnel expenses									2,251.0	2,212.1	2,143.4
Advertising expenses									132.9	138.3	140.0
Rent & related expenses									448.8	432.6	402.7
Other expenses									931.3	902.7	867.6
Depreciation									172.3	161.3	143.7
Floor plan interest expense									170.6	193.1	135.3
Other interest expense									91.6	87.8	92.6
Equity in earnings of affiliates									192.9	200.7	293.7
Segment income									1,263.7	1,289.9	1,475.1
Gain on sale of dealership									52.3	0.0	0.0
Goodwill impairment charges									0.0	0.0	40.7
Retail Automotive
Segment Reporting Information [Line Items]
Revenues									27,474.6	27,565.8	26,598.2
Total cost of sales									22,992.2	23,111.4	22,208.4
Selling, general, and administrative expenses:
Personnel expenses									1,921.3	1,875.6	1,817.5
Advertising expenses									127.7	133.3	134.9
Rent & related expenses									406.9	393.0	364.0
Other expenses									858.8	825.9	784.6
Depreciation									150.1	140.7	127.4
Floor plan interest expense									145.9	164.2	118.2
Other interest expense									76.8	82.1	90.3
Equity in earnings of affiliates									0.6	3.1	3.9
Segment income									847.8	842.7	916.1
Retail Commercial Truck
Segment Reporting Information [Line Items]
Revenues									3,411.3	3,521.1	3,684.3
Total cost of sales									2,869.0	2,936.6	3,091.9
Selling, general, and administrative expenses:
Personnel expenses									254.7	264.3	253.6
Advertising expenses									3.4	3.2	3.3
Rent & related expenses									26.8	25.7	25.8
Other expenses									49.6	53.4	57.3
Depreciation									16.2	15.1	11.6
Floor plan interest expense									16.9	19.2	14.5
Other interest expense									0.0	0.0	1.3
Equity in earnings of affiliates									0.0	0.0	0.0
Segment income									174.7	203.6	225.0
Other
Segment Reporting Information [Line Items]
Revenues									922.6	777.9	634.0
Total cost of sales									730.3	599.7	468.8
Selling, general, and administrative expenses:
Personnel expenses									75.0	72.2	72.3
Advertising expenses									1.8	1.8	1.8
Rent & related expenses									15.1	13.9	12.9
Other expenses									22.9	23.4	25.7
Depreciation									6.0	5.5	4.7
Floor plan interest expense									7.8	9.7	2.6
Other interest expense									14.8	5.7	1.0
Equity in earnings of affiliates									0.0	0.0	0.0
Segment income									48.9	46.0	44.2
Non-Automotive Investments
Segment Reporting Information [Line Items]
Revenues									0.0	0.0	0.0
Total cost of sales									0.0	0.0	0.0
Selling, general, and administrative expenses:
Personnel expenses									0.0	0.0	0.0
Advertising expenses									0.0	0.0	0.0
Rent & related expenses									0.0	0.0	0.0
Other expenses									0.0	0.0	0.0
Depreciation									0.0	0.0	0.0
Floor plan interest expense									0.0	0.0	0.0
Other interest expense									0.0	0.0	0.0
Equity in earnings of affiliates									192.3	197.6	289.8
Segment income									$ 192.3	$ 197.6	$ 289.8