Segment Information (Details) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segments	Jun. 30, 2014 USD ($) segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Number of reportable segments | segments				3	3
Total consolidated revenue		$ 5,224.3	$ 4,788.5	$ 10,168.5	$ 9,152.0
Total segment income	[1]	259.4	233.8	507.8	446.3
Corporate and other		(51.5)	(49.3)	(98.2)	(85.9)
Other interest expense		(21.6)	(21.3)	(43.0)	(42.9)
Interest income		0.0	0.1	0.1	0.1
Other income, net		0.5	0.9	1.6	2.4
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES		186.8	164.2	368.3	320.0
AN Reportable Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue		5,192.2	4,754.6	10,099.8	9,083.4
AN Reportable Segment, Domestic [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue		1,764.2	1,604.9	3,429.9	3,077.9
Total segment income	[1]	84.9	70.5	164.2	134.3
AN Reportable Segment, Import [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue		1,795.0	1,717.8	3,473.7	3,267.2
Total segment income	[1]	80.1	77.5	155.1	142.9
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue		1,633.0	1,431.9	3,196.2	2,738.3
Total segment income	[1]	94.4	85.8	188.5	169.1
Corporate and Other [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue		$ 32.1	$ 33.9	$ 68.7	$ 68.6

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