Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
TOTAL REVENUE		$ 21,412.8	$ 21,534.6	$ 21,609.0
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		18,015.5	18,175.6	18,295.8
Gross Profit:
TOTAL GROSS PROFIT		3,397.3	3,359.0	3,313.2
Selling, general, and administrative expenses		2,509.8	2,436.2	2,349.4
Depreciation and amortization		166.2	158.6	143.4
Franchise rights impairment		8.1	0.0	0.0
Other income, net		(64.7)	(79.2)	(69.1)
OPERATING INCOME		777.9	843.4	889.5
Non-operating income (expense) items:
Floorplan interest expense		(130.4)	(97.0)	(76.5)
Other interest expense		(119.4)	(120.2)	(115.5)
Interest income		1.1	1.0	1.1
Other income, net		0.2	9.3	3.7
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES		529.4	636.5	702.3
Income tax provision		133.5	201.5	270.6
NET INCOME FROM CONTINUING OPERATIONS		395.9	435.0	431.7
Income (loss) from discontinued operations, net of income taxes		0.1	(0.4)	(1.2)
NET INCOME		$ 396.0	$ 434.6	$ 430.5
BASIC EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 4.36	$ 4.45	$ 4.19
Discontinued operations (in dollars per share)	[1]	0	0	(0.01)
Net income (in dollars per share)	[1]	$ 4.36	$ 4.44	$ 4.18
Weighted average common shares outstanding (in shares)		90.9	97.8	103.1
DILUTED EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 4.34	$ 4.43	$ 4.16
Discontinued operations (in dollars per share)	[1]	0	0	(0.01)
Net income (in dollars per share)	[1]	$ 4.34	$ 4.43	$ 4.15
Weighted average common shares outstanding (in shares)		91.3	98.2	103.8
COMMON SHARES OUTSTANDING, net of treasury stock, at period end (in shares)		90.0	91.6	100.7
New Vehicle [Member]
Revenue:
TOTAL REVENUE		$ 11,751.6	$ 12,180.8	$ 12,255.8
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		11,235.5	11,592.4	11,620.0
Gross Profit:
TOTAL GROSS PROFIT		516.1	588.4	635.8
Used Vehicle [Member]
Revenue:
TOTAL REVENUE		5,123.3	4,878.4	4,995.3
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		4,781.6	4,563.2	4,677.7
Gross Profit:
TOTAL GROSS PROFIT		341.7	315.2	317.6
Parts and Service [Member]
Revenue:
TOTAL REVENUE		3,447.6	3,398.3	3,321.4
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		1,892.3	1,907.6	1,886.7
Gross Profit:
TOTAL GROSS PROFIT		1,555.3	1,490.7	1,434.7
Finance and Insurance, Net [Member]
Revenue:
TOTAL REVENUE		981.4	939.2	894.6
Gross Profit:
TOTAL GROSS PROFIT		981.4	939.2	894.6
Product and Service, Other [Member]
Revenue:
TOTAL REVENUE		108.9	137.9	141.9
Cost of Sales:
TOTAL COST OF SALES (excluding depreciation shown below)		106.1	112.4	111.4
Gross Profit:
TOTAL GROSS PROFIT		$ 2.8	$ 25.5	$ 30.5

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