Goodwill And Intangible Assets, Net (Goodwill Allocated to Reporting Units and Changes in Fair Value (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]			$ 1,501.9
Acquisitions, dispositions, and other adjustments, net	[2]			(2.3)
Impairment		$ 0.0	$ 0.0	(318.3)	$ 0.0
Goodwill, ending balance	[1],[3]	1,181.3		1,181.3
Accumulated impairment loss		1,470.0		1,470.0		$ 1,470.0
Reporting Unit, Domestic [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]			227.3
Acquisitions, dispositions, and other adjustments, net	[2]			(0.1)
Impairment				0.0
Goodwill, ending balance	[1],[3]	227.2		227.2
Accumulated impairment loss		140.0		140.0		$ 140.0
Reporting Unit, Import [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]			498.9
Acquisitions, dispositions, and other adjustments, net	[2]			(2.0)
Impairment				0.0
Goodwill, ending balance	[1],[3]	496.9		496.9
Reporting Unit, Premium Luxury [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]			714.9
Acquisitions, dispositions, and other adjustments, net	[2]			(0.3)
Impairment				(257.4)
Goodwill, ending balance	[1],[3]	457.2		457.2
Accumulated impairment loss		257.4		257.4
Reporting Unit, Collision Centers [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]			41.7
Acquisitions, dispositions, and other adjustments, net	[2]			(0.1)
Impairment				(41.6)
Goodwill, ending balance	[1],[3]	0.0		0.0
Accumulated impairment loss		41.6		41.6
Reporting Unit, Parts Centers [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	[1]			19.1
Acquisitions, dispositions, and other adjustments, net	[2]			0.2
Impairment				(19.3)
Goodwill, ending balance	[1],[3]	0.0		0.0
Accumulated impairment loss		$ 19.3		$ 19.3

[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 Unaudited Condensed Consolidated Balance Sheet as of period end.

[3] Net of accumulated impairment losses of $257.4 million associated with our Premium Luxury reporting unit, $41.6 million associated with our Collision Centers reporting unit, and $19.3 million associated with our Parts Centers reporting unit, each of which were recorded during the three months ended March 31, 2020.