Segment Information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Revenues and segment income by reportable segment
Revenue	$ 4,471.1	$ 4,015.2	$ 16,600.0
Segment income	115.9	102.3
Intersegment Elimination
Revenues and segment income by reportable segment
Revenue	(0.3)	(0.7)
Retail Automotive | Operating segments
Revenues and segment income by reportable segment
Revenue	4,175.0	3,919.2
Segment income	103.1	93.9
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Revenue	192.7
Segment income	7.4	0.6
Other | Operating segments
Revenues and segment income by reportable segment
Revenue	103.7	96.7
Segment income	$ 5.4	$ 7.8