Revenues - Retail Automotive Dealership (Details) - Retail Automotive Dealership - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues
Total revenues	$ 5,296.0
U.S.
Revenues
Total revenues	2,750.9
UK
Revenues
Total revenues	2,192.8
Germany and Italy
Revenues
Total revenues	352.3
New vehicle
Revenues
Total revenues	2,446.8
Used vehicle
Revenues
Total revenues	1,866.8
Finance and insurance, net
Revenues
Total revenues	160.8
Service and parts
Revenues
Total revenues	543.5
Fleet and wholesale
Revenues
Total revenues	$ 278.1
Without Adoption of ASC 606
Revenues
Total revenues		$ 4,756.4
Without Adoption of ASC 606 | U.S.
Revenues
Total revenues		2,656.2
Without Adoption of ASC 606 | UK
Revenues
Total revenues		1,826.4
Without Adoption of ASC 606 | Germany and Italy
Revenues
Total revenues		273.8
Without Adoption of ASC 606 | New vehicle
Revenues
Total revenues		2,307.4
Without Adoption of ASC 606 | Used vehicle
Revenues
Total revenues		1,541.0
Without Adoption of ASC 606 | Finance and insurance, net
Revenues
Total revenues		137.4
Without Adoption of ASC 606 | Service and parts
Revenues
Total revenues		498.9
Without Adoption of ASC 606 | Fleet and wholesale
Revenues
Total revenues		$ 271.7