Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013 franchises		Sep. 30, 2012		Sep. 30, 2013 segments franchises		Sep. 30, 2012 segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Number of operating segments					3		3
Number of reportable segments					3		3
Owned and operated new vehicle franchises	267				267
Total revenues	$ 4,470.8		$ 3,933.8		$ 12,993.7		$ 11,495.3
Total segment income	211.1	[1]	183.8	[1]	624.2	[1]	543.3	[1]
Corporate and other	(36.6)		(31.5)		(125.9)		(99.6)
Other interest expense	(22.3)		(22.2)		(66.6)		(65.2)
Interest income	0.1		0		0.2		0.2
Other income (loss), net	(0.7)		2.5		2.2		3.1
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	151.6		132.6		434.1		381.8
Operating Segments [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	4,434.1		3,894.7		12,875.7		11,376.9
Operating Segments, Domestic [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	1,491.2		1,285.0		4,384.5		3,809.2
Total segment income	64.2	[1]	51.9	[1]	188.9	[1]	155.2	[1]
Operating Segments, Import [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	1,695.5		1,523.3		4,828.5		4,365.7
Total segment income	72.7	[1]	69.0	[1]	216.6	[1]	198.1	[1]
Operating Segments, Premium Luxury [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	1,247.4		1,086.4		3,662.7		3,202.0
Total segment income	74.2	[1]	62.9	[1]	218.7	[1]	190.0	[1]
Corporate and Other [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total revenues	$ 36.7		$ 39.1		$ 118.0		$ 118.4

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