Revenues - Retail Automotive Dealership (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Revenues
Total revenues	$ 6,987.5	$ 3,651.1	$ 12,761.3	$ 8,660.2
Retail automotive dealership
Revenues
Total revenues	6,197.6	3,153.5	11,404.5	7,570.1	$ 17,900.0
Retail automotive dealership | U.S.
Revenues
Total revenues	3,614.5	2,053.8	6,619.3	4,507.7
Retail automotive dealership | UK
Revenues
Total revenues	2,190.6	837.8	4,044.0	2,503.2
Retail automotive dealership | Germany and Italy
Revenues
Total revenues	392.5	261.9	741.2	559.2
New vehicle | Retail automotive dealership
Revenues
Total revenues	2,811.3	1,384.7	5,232.7	3,249.2
Used vehicle | Retail automotive dealership
Revenues
Total revenues	2,327.6	1,166.0	4,135.6	2,785.6
Finance and insurance, net | Retail automotive dealership
Revenues
Total revenues	212.3	97.1	381.1	241.5
Service and parts | Retail automotive dealership
Revenues
Total revenues	546.2	345.2	1,049.4	858.5
Fleet and wholesale | Retail automotive dealership
Revenues
Total revenues	$ 300.2	$ 160.5	$ 605.7	$ 435.3