Segment Information (Details) $ in Millions		12 Months Ended
		Dec. 31, 2019 USD ($) segments		Dec. 31, 2018 USD ($) segments		Dec. 31, 2017 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments		3		3		3
Revenues		$ 21,335.7		$ 21,412.8		$ 21,534.6
Floorplan interest expense		138.4		130.4		97.0
Depreciation and amortization		180.5		166.2		158.6
Capital expenditures		257.4		393.6		332.9
Segment assets		10,543.3		10,665.1
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	685.2		647.5		746.4
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
AN Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenues		20,574.9		20,931.8		21,158.9
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	957.3		894.9		909.0
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		6,671.4		7,134.5		7,452.8
Floorplan interest expense		51.8		51.3		40.9
Depreciation and amortization		37.2		37.3		38.2
Capital expenditures		38.6		77.7		36.2
Segment assets		2,483.6		2,684.5
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	257.6		249.3		257.1
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		6,468.7		6,786.4		6,873.4
Floorplan interest expense		30.7		31.0		23.2
Depreciation and amortization		33.4		33.2		34.3
Capital expenditures		23.2		56.2		32.8
Segment assets		1,842.1		1,934.3
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	318.6		304.7		303.1
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		7,434.8		7,010.9		6,832.7
Floorplan interest expense		47.3		41.7		28.4
Depreciation and amortization		53.5		47.6		44.5
Capital expenditures		145.3		144.2		101.7
Segment assets		3,150.5		3,046.4
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	381.1		340.9		348.8
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues		760.8	[2]	481.0	[2]	375.7
Floorplan interest expense		8.6		6.4		4.5
Depreciation and amortization		56.4		48.1		41.6
Capital expenditures		50.3		115.5		162.2
Segment assets		3,067.1		2,999.9
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	$ (272.1)		$ (247.4)		$ (162.6)

[1] Segment income is defined as operating income less floorplan interest expense.

[2] “Corporate and other” is comprised of our other businesses, including collision centers, auction operations, AutoNation USA stand-alone used vehicle sales and service centers, and aftermarket collision parts businesses.