Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenue:
TOTAL REVENUE		$ 26,948.9	$ 26,985.0	$ 25,844.0
Cost of Sales:
TOTAL COST OF SALES		21,817.4	21,719.7	20,891.4
Gross Profit:
TOTAL GROSS PROFIT		5,131.5	5,265.3	4,952.6
Selling, general, and administrative expenses		3,253.2	3,026.1	2,876.2
Depreciation and amortization		220.5	200.3	193.3
Other (income) expense, net		5.9	14.4	(19.7)
OPERATING INCOME		1,651.9	2,024.5	1,902.8
Non-operating income (expense) items:
Floorplan interest expense		(144.7)	(41.4)	(25.7)
Other interest expense		(181.4)	(134.9)	(93.0)
Other income (loss), net		24.4	(14.7)	24.3
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES		1,350.2	1,833.5	1,808.4
Income tax provision		330.0	455.8	435.1
NET INCOME FROM CONTINUING OPERATIONS		1,020.2	1,377.7	1,373.3
Income (loss) from discontinued operations, net of income taxes		0.9	(0.3)	(0.3)
NET INCOME		$ 1,021.1	$ 1,377.4	$ 1,373.0
BASIC EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 22.87	$ 24.47	$ 18.51
Discontinued operations (in dollars per share)	[1]	0.02	(0.01)	0
Net income (in dollars per share)	[1]	$ 22.89	$ 24.47	$ 18.50
Weighted average common shares outstanding, basic (in shares)		44.6	56.3	74.2
DILUTED EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 22.72	$ 24.30	$ 18.31
Discontinued operations (in dollars per share)	[1]	0.02	(0.01)	0
Net income (in dollars per share)	[1]	$ 22.74	$ 24.29	$ 18.31
Weighted average common shares outstanding (in shares)		44.9	56.7	75.0
COMMON SHARES OUTSTANDING, net of treasury stock, at period end (in shares)		41.6	47.6	62.6
New Vehicle [Member]
Revenue:
TOTAL REVENUE		$ 12,767.4	$ 11,754.4	$ 12,081.7
Cost of Sales:
TOTAL COST OF SALES		11,705.6	10,387.8	10,880.1
Gross Profit:
TOTAL GROSS PROFIT		1,061.8	1,366.6	1,201.6
Used Vehicle [Member]
Revenue:
TOTAL REVENUE		8,198.5	9,661.8	8,638.8
Cost of Sales:
TOTAL COST OF SALES		7,690.5	9,108.7	7,950.7
Gross Profit:
TOTAL GROSS PROFIT		508.0	553.1	688.1
Parts and Service [Member]
Revenue:
TOTAL REVENUE		4,533.7	4,100.6	3,706.6
Cost of Sales:
TOTAL COST OF SALES		2,394.4	2,200.3	2,033.9
Gross Profit:
TOTAL GROSS PROFIT		2,139.3	1,900.3	1,672.7
Finance and Insurance, Net [Member]
Revenue:
TOTAL REVENUE		1,418.8	1,437.3	1,384.5
Gross Profit:
TOTAL GROSS PROFIT		1,418.8	1,437.3	1,384.5
Product and Service, Other [Member]
Revenue:
TOTAL REVENUE		30.5	30.9	32.4
Cost of Sales:
TOTAL COST OF SALES		26.9	22.9	26.7
Gross Profit:
TOTAL GROSS PROFIT		$ 3.6	$ 8.0	$ 5.7

[1] (1) EPS amounts are calculated discretely and, therefore, may not add up to the total due to rounding.