Goodwill and Franchise Value (Schedule of Goodwill) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2008
Goodwill [Roll Forward]
Beginning balance		$ 454.6	$ 434.9
Adjustments to purchase price allocations			5.1
Additions through acquisitions		141.9	17.4
Reductions from impairments	$ (3.5)		(1.7)
Reductions through divestitures		(0.8)	(1.1)
Ending balance		592.2	454.6
Accumulated impairment loss				$ 299.3
Goodwill acquired including purchase accounting adjustments			22.5
Domestic
Goodwill [Roll Forward]
Beginning balance		171.8	164.5
Adjustments to purchase price allocations			1.6
Additions through acquisitions		32.0	6.2
Reductions from impairments		(0.5)	(0.3)
Reductions through divestitures		(0.1)	(0.2)
Ending balance		203.2	171.8
Import
Goodwill [Roll Forward]
Beginning balance		197.3	188.9
Adjustments to purchase price allocations			1.6
Additions through acquisitions		92.9	9.0
Reductions from impairments		(3.0)	(1.3)
Reductions through divestitures		(0.7)	(0.9)
Ending balance		286.5	197.3
Luxury
Goodwill [Roll Forward]
Beginning balance		85.5	81.5
Adjustments to purchase price allocations			1.9
Additions through acquisitions		17.0	2.2
Reductions from impairments		0.0	(0.1)
Reductions through divestitures		0.0	0.0
Ending balance		$ 102.5	$ 85.5