Goodwill and Franchise Value (Schedule of Goodwill) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2008
Goodwill [Roll Forward]
Beginning balance	$ 593.0	$ 454.6
Additions through acquisitions	24.5	144.9
Reductions through divestitures		(3.0)
Reductions from impairments		(3.5)
Ending balance	617.5	593.0
Accumulated impairment loss			$ 299.3
Domestic
Goodwill [Roll Forward]
Beginning balance	204.5	171.8
Additions through acquisitions	5.5	33.3
Reductions through divestitures		(0.1)
Reductions from impairments		(0.5)
Ending balance	210.0	204.5
Import
Goodwill [Roll Forward]
Beginning balance	287.9	197.3
Additions through acquisitions	10.7	94.3
Reductions through divestitures		(0.7)
Reductions from impairments		(3.0)
Ending balance	298.6	287.9
Luxury
Goodwill [Roll Forward]
Beginning balance	100.6	85.5
Additions through acquisitions	8.3	17.3
Reductions through divestitures		(2.2)
Reductions from impairments		0.0
Ending balance	$ 108.9	$ 100.6