Revenues - Retail Automotive Dealership (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenues
Total revenues	$ 5,755.8	$ 5,940.3	$ 11,320.2	$ 11,687.2
Retail Automotive Dealership
Revenues
Total revenues	5,196.3	5,455.5	10,287.5	10,751.5	$ 20,800.0
Retail Automotive Dealership | U.S.
Revenues
Total revenues	2,905.9	2,953.9	5,628.6	5,704.8
Retail Automotive Dealership | UK
Revenues
Total revenues	1,928.2	2,107.0	3,968.1	4,299.8
Retail Automotive Dealership | Germany and Italy
Revenues
Total revenues	362.2	394.6	690.8	746.9
New vehicle | Retail Automotive Dealership
Revenues
Total revenues	2,310.4	2,528.6	4,541.6	4,975.4
Used vehicle | Retail Automotive Dealership
Revenues
Total revenues	1,852.7	1,896.9	3,704.7	3,763.7
Finance and insurance, net | Retail Automotive Dealership
Revenues
Total revenues	165.5	162.9	325.5	323.7
Service and parts | Retail Automotive Dealership
Revenues
Total revenues	550.7	547.8	1,110.5	1,091.3
Fleet and wholesale | Retail Automotive Dealership
Revenues
Total revenues	$ 317.0	$ 319.3	$ 605.2	$ 597.4