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Goodwill and Intangible Assets, Net (Goodwill) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 1,501,900,000	$ 1,501,900,000	$ 1,513,200,000
Acquisitions, dispositions, and other adjustments, net		1,400,000	(11,300,000)
Gain/(Loss) on goodwill impairment		(318,300,000)	0	$ 0
Goodwill, ending balance		1,185,000,000.0	1,501,900,000	1,513,200,000
Accumulated impairment losses		1,470,000,000	1,470,000,000	1,470,000,000
Nonrecurring [Member]
Goodwill [Roll Forward]
Gain/(Loss) on goodwill impairment	(318,300,000)	(318,300,000)	0
Reporting Unit, Domestic [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	227,300,000	227,300,000	232,500,000
Acquisitions, dispositions, and other adjustments, net		(100,000)	(5,200,000)
Gain/(Loss) on goodwill impairment		0
Goodwill, ending balance		227,200,000	227,300,000	232,500,000
Accumulated impairment losses		140,000,000.0	140,000,000.0	140,000,000.0
Reporting Unit, Import [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	498,900,000	498,900,000	520,900,000
Acquisitions, dispositions, and other adjustments, net		1,700,000	(22,000,000.0)
Gain/(Loss) on goodwill impairment		0
Goodwill, ending balance		500,600,000	498,900,000	520,900,000
Reporting Unit, Premium Luxury [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	714,900,000	714,900,000	717,700,000
Acquisitions, dispositions, and other adjustments, net		(300,000)	(2,800,000)
Goodwill, ending balance		457,200,000	714,900,000	717,700,000
Accumulated impairment losses		257,400,000
Reporting Unit, Premium Luxury [Member] | Nonrecurring [Member]
Goodwill [Roll Forward]
Gain/(Loss) on goodwill impairment	(257,400,000)
Reporting Unit, Collision Centers [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	41,700,000	41,700,000	42,100,000
Acquisitions, dispositions, and other adjustments, net		(100,000)	(400,000)
Goodwill, ending balance		0	41,700,000	42,100,000
Accumulated impairment losses		41,600,000
Reporting Unit, Collision Centers [Member] | Nonrecurring [Member]
Goodwill [Roll Forward]
Gain/(Loss) on goodwill impairment	(41,600,000)
Reporting Unit, Parts Centers [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	19,100,000	19,100,000	0
Acquisitions, dispositions, and other adjustments, net		200,000	19,100,000
Goodwill, ending balance		0	$ 19,100,000	$ 0
Accumulated impairment losses		$ 19,300,000
Reporting Unit, Parts Centers [Member] | Nonrecurring [Member]
Goodwill [Roll Forward]
Gain/(Loss) on goodwill impairment	$ (19,300,000)

X

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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