Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014 segments		Jun. 30, 2013 segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Number of reportable segments					3		3
Total consolidated revenue	$ 4,788.5		$ 4,426.5		$ 9,152.0		$ 8,522.9
Total segment income	233.8	[1]	214.7	[1]	446.3	[1]	413.1	[1]
Corporate and other	(49.3)		(47.4)		(85.9)		(89.3)
Other interest expense	(21.3)		(22.0)		(42.9)		(44.3)
Interest income	0.1		0		0.1		0.1
Other income, net	0.9		1.3		2.4		2.9
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	164.2		146.6		320.0		282.5
AN Reportable Segment, Domestic [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue	1,604.9		1,515.3		3,077.9		2,893.3
Total segment income	70.5	[1]	66.1	[1]	134.3	[1]	124.7	[1]
AN Reportable Segment, Import [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue	1,717.8		1,629.3		3,267.2		3,133.0
Total segment income	77.5	[1]	72.9	[1]	142.9	[1]	143.9	[1]
AN Reportable Segment, Premium Luxury [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue	1,431.9		1,242.4		2,738.3		2,415.3
Total segment income	85.8	[1]	75.7	[1]	169.1	[1]	144.5	[1]
AN Reportable Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue	4,754.6		4,387.0		9,083.4		8,441.6
Corporate and Other [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total consolidated revenue	$ 33.9		$ 39.5		$ 68.6		$ 81.3

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