Revenues - Retail Automotive Dealership (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenues
Total revenues	$ 5,564.4	$ 5,746.9
Retail Automotive Dealership
Revenues
Total revenues	5,091.2	5,296.0	$ 20,800.0
Retail Automotive Dealership | U.S.
Revenues
Total revenues	2,722.8	2,750.9
Retail Automotive Dealership | UK
Revenues
Total revenues	2,039.9	2,192.8
Retail Automotive Dealership | Germany and Italy
Revenues
Total revenues	328.5	352.3
New vehicle | Retail Automotive Dealership
Revenues
Total revenues	2,231.2	2,446.8
Used vehicle | Retail Automotive Dealership
Revenues
Total revenues	1,852.0	1,866.8
Finance and insurance, net | Retail Automotive Dealership
Revenues
Total revenues	160.0	160.8
Service and parts | Retail Automotive Dealership
Revenues
Total revenues	559.8	543.5
Fleet and wholesale | Retail Automotive Dealership
Revenues
Total revenues	$ 288.2	$ 278.1