Goodwill and Intangible Assets, Net (Goodwill) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]	$ 1,513.2	$ 1,515.0
Acquisitions, dispositions, and other adjustments, net	[2]	(11.3)	(1.8)
Goodwill, ending balance	[1]	1,501.9	1,513.2
Accumulated impairment losses		1,470.0	1,470.0	$ 1,470.0
Reporting Unit, Domestic [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]	232.5	231.7
Acquisitions, dispositions, and other adjustments, net	[2]	(5.2)	0.8
Goodwill, ending balance	[1]	227.3	232.5
Accumulated impairment losses		140.0	140.0	$ 140.0
Reporting Unit, Import [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]	520.9	532.4
Acquisitions, dispositions, and other adjustments, net	[2]	(22.0)	(11.5)
Goodwill, ending balance	[1]	498.9	520.9
Reporting Unit, Premium Luxury [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]	717.7	712.1
Acquisitions, dispositions, and other adjustments, net	[2]	(2.8)	5.6
Goodwill, ending balance	[1]	714.9	717.7
Reporting Unit, Collision Centers [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]	42.1	38.8
Acquisitions, dispositions, and other adjustments, net	[2]	(0.4)	3.3
Goodwill, ending balance	[1]	41.7	42.1
Reporting Unit, Parts Distribution Centers [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]	0.0	0.0
Acquisitions, dispositions, and other adjustments, net	[2]	19.1	0.0
Goodwill, ending balance	[1]	$ 19.1	$ 0.0

[1] Net of accumulated impairment losses of $1.47 billion associated with our single reporting unit (prior to September 30, 2008, our reporting unit structure was comprised of a single reporting unit) and $140.0 million associated with our Domestic reporting unit, both of which were recorded during the year ended December 31, 2008.

[2] Includes amounts reclassified to held for sale and related adjustments, which are presented in Other Current Assets in our Consolidated Balance Sheet as of period end.