Revenues - Retail Automotive Dealership (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Revenues
Total revenues	$ 5,967.6	$ 5,658.6	$ 17,287.8	$ 17,345.8
Retail Automotive Dealership
Revenues
Total revenues	5,155.4	5,148.5	15,442.9	15,900.0	$ 20,800.0
Retail Automotive Dealership | U.S.
Revenues
Total revenues	2,995.5	2,891.6	8,624.1	8,596.4
Retail Automotive Dealership | UK
Revenues
Total revenues	1,859.6	1,945.1	5,827.7	6,244.9
Retail Automotive Dealership | Germany and Italy
Revenues
Total revenues	300.3	311.8	991.1	1,058.7
New vehicle | Retail Automotive Dealership
Revenues
Total revenues	2,332.3	2,350.2	6,873.9	7,325.6
Used vehicle | Retail Automotive Dealership
Revenues
Total revenues	1,824.5	1,825.2	5,529.2	5,588.9
Finance and insurance, net | Retail Automotive Dealership
Revenues
Total revenues	166.0	158.5	491.5	482.2
Service and parts | Retail Automotive Dealership
Revenues
Total revenues	543.5	523.8	1,654.0	1,615.1
Fleet and wholesale | Retail Automotive Dealership
Revenues
Total revenues	$ 289.1	$ 290.8	$ 894.3	$ 888.2