Segment Information (Details) $ in Millions		3 Months Ended
		Mar. 31, 2018 USD ($) segments	Mar. 31, 2017 USD ($) segments
Segment Reporting Information [Line Items]
Number of reportable segments | segments		3	3
Revenues		$ 5,259.9	$ 5,139.4
Segment income		220.8	213.6
AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		1,773.7	1,800.8
Segment income	[1]	60.3	61.4
AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		1,652.6	1,631.7
Segment income	[1]	72.8	71.7
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		1,717.8	1,616.6
Segment income	[1]	87.7	80.5
Intercompany Revenue Eliminated [Member] | AN Reportable Segment, Domestic [Member]
Segment Reporting Information [Line Items]
Revenues		(140.6)	(138.8)
Intercompany Revenue Eliminated [Member] | AN Reportable Segment, Import [Member]
Segment Reporting Information [Line Items]
Revenues		(80.9)	(92.0)
Intercompany Revenue Eliminated [Member] | AN Reportable Segment, Premium Luxury [Member]
Segment Reporting Information [Line Items]
Revenues		$ (102.1)	$ (116.0)

[1] Segment income represents income for each of our reportable segments and is defined as operating income less floorplan interest expense.