Segment Information (Details)		12 Months Ended
		Dec. 31, 2018 USD ($) segments		Dec. 31, 2017 USD ($) segments	Dec. 31, 2016 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments		3		3	3
Revenues		$ 21,412,800,000		$ 21,534,600,000	$ 21,609,000,000.0
Floorplan interest expense		130,400,000		97,000,000.0	76,500,000
Depreciation and amortization		166,200,000		158,600,000	143,400,000
Capital expenditures		393,600,000		332,900,000	253,200,000
Segment assets		10,665,100,000		10,271,500,000
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	647,500,000		746,400,000	813,000,000.0
Other interest expense		(119,400,000)		(120,200,000)	(115,500,000)
Interest income		1,100,000		1,000,000.0	1,100,000
Other income, net		200,000		9,300,000	3,700,000
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES		529,400,000		636,500,000	702,300,000
AN Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenues		20,931,800,000		21,158,900,000	21,361,400,000
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	894,900,000		909,000,000.0	958,100,000
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		7,134,500,000		7,452,800,000	7,810,000,000.0
Floorplan interest expense		51,300,000		40,900,000	33,700,000
Depreciation and amortization		37,300,000		38,200,000	37,500,000
Capital expenditures		77,700,000		36,200,000	62,500,000
Segment assets		2,684,500,000		2,563,900,000
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	249,300,000		257,100,000	311,100,000
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		6,786,400,000		6,873,400,000	6,886,100,000
Floorplan interest expense		31,000,000.0		23,200,000	17,400,000
Depreciation and amortization		33,200,000		34,300,000	35,400,000
Capital expenditures		56,200,000		32,800,000	28,000,000.0
Segment assets		1,934,300,000		1,992,600,000
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	304,700,000		303,100,000	296,800,000
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		7,010,900,000		6,832,700,000	6,665,300,000
Floorplan interest expense		41,700,000		28,400,000	22,700,000
Depreciation and amortization		47,600,000		44,500,000	40,700,000
Capital expenditures		144,200,000		101,700,000	95,600,000
Segment assets		3,046,400,000		2,716,800,000
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	340,900,000		348,800,000	350,200,000
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues		481,000,000.0	[2]	375,700,000	247,600,000
Floorplan interest expense		6,400,000		4,500,000	2,700,000
Depreciation and amortization		48,100,000		41,600,000	29,800,000
Capital expenditures		115,500,000		162,200,000	67,100,000
Segment assets		2,999,900,000		2,998,200,000
Segment Reporting Information, Income (Loss) before Income Taxes [Abstract]
Segment income (loss)	[1]	$ (247,400,000)		$ (162,600,000)	$ (145,100,000)

[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.