Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue:
TOTAL REVENUE		$ 21,335.7	$ 21,412.8	$ 21,534.6
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		17,812.7	18,015.5	18,175.6
Gross Profit:
TOTAL GROSS PROFIT		3,523.0	3,397.3	3,359.0
Selling, general, and administrative expenses		2,558.6	2,509.8	2,436.2
Depreciation and amortization		180.5	166.2	158.6
Franchise rights impairment		9.6	8.1	0.0
Other income, net		(49.3)	(64.7)	(79.2)
OPERATING INCOME		823.6	777.9	843.4
Non-operating income (expense) items:
Floorplan interest expense		(138.4)	(130.4)	(97.0)
Other interest expense		(106.7)	(119.4)	(120.2)
Interest income		0.5	1.1	1.0
Other income, net		33.6	0.2	9.3
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES		612.6	529.4	636.5
Income tax provision		161.8	133.5	201.5
NET INCOME FROM CONTINUING OPERATIONS		450.8	395.9	435.0
Income (loss) from discontinued operations, net of income taxes		(0.8)	0.1	(0.4)
NET INCOME		$ 450.0	$ 396.0	$ 434.6
BASIC EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 5.00	$ 4.36	$ 4.45
Discontinued operations (in dollars per share)	[1]	(0.01)	0	0
Net income (in dollars per share)	[1]	$ 4.99	$ 4.36	$ 4.44
Weighted average common shares outstanding (in shares)		90.1	90.9	97.8
DILUTED EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 4.98	$ 4.34	$ 4.43
Discontinued operations (in dollars per share)	[1]	(0.01)	0	0
Net income (in dollars per share)	[1]	$ 4.97	$ 4.34	$ 4.43
Weighted average common shares outstanding (in shares)		90.5	91.3	98.2
COMMON SHARES OUTSTANDING, net of treasury stock, at period end (in shares)		89.3	90.0	91.6
New Vehicle [Member]
Revenue:
TOTAL REVENUE		$ 11,166.5	$ 11,751.6	$ 12,180.8
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		10,662.6	11,235.5	11,592.4
Gross Profit:
TOTAL GROSS PROFIT		503.9	516.1	588.4
Used Vehicle [Member]
Revenue:
TOTAL REVENUE		5,466.5	5,123.3	4,878.4
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		5,098.5	4,781.6	4,563.2
Gross Profit:
TOTAL GROSS PROFIT		368.0	341.7	315.2
Parts and Service [Member]
Revenue:
TOTAL REVENUE		3,572.1	3,447.6	3,398.3
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		1,949.5	1,892.3	1,907.6
Gross Profit:
TOTAL GROSS PROFIT		1,622.6	1,555.3	1,490.7
Finance and Insurance, Net [Member]
Revenue:
TOTAL REVENUE		1,023.3	981.4	939.2
Gross Profit:
TOTAL GROSS PROFIT		1,023.3	981.4	939.2
Product and Service, Other [Member]
Revenue:
TOTAL REVENUE		107.3	108.9	137.9
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		102.1	106.1	112.4
Gross Profit:
TOTAL GROSS PROFIT		$ 5.2	$ 2.8	$ 25.5

[1] Earnings per share amounts are calculated discretely and therefore may not add up to the total due to rounding.