Revenues - Retail Automotive Dealership (Details) - Retail Automotive Dealership - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Total revenues	$ 5,455.5		$ 10,751.5
U.S.
Revenues
Total revenues	2,953.9		5,704.8
UK
Revenues
Total revenues	2,107.0		4,299.8
Germany and Italy
Revenues
Total revenues	394.6		746.9
New vehicle
Revenues
Total revenues	2,528.6		4,975.4
Used vehicle
Revenues
Total revenues	1,896.9		3,763.7
Finance and insurance, net
Revenues
Total revenues	162.9		323.7
Service and parts
Revenues
Total revenues	547.8		1,091.3
Fleet and wholesale
Revenues
Total revenues	$ 319.3		$ 597.4
Without Adoption of ASC 606
Revenues
Total revenues		$ 5,040.7		$ 9,797.1
Without Adoption of ASC 606 | U.S.
Revenues
Total revenues		3,016.4		5,672.5
Without Adoption of ASC 606 | UK
Revenues
Total revenues		1,721.3		3,547.7
Without Adoption of ASC 606 | Germany and Italy
Revenues
Total revenues		303.0		576.9
Without Adoption of ASC 606 | New vehicle
Revenues
Total revenues		2,401.7		4,709.1
Without Adoption of ASC 606 | Used vehicle
Revenues
Total revenues		1,640.1		3,181.1
Without Adoption of ASC 606 | Finance and insurance, net
Revenues
Total revenues		147.2		284.6
Without Adoption of ASC 606 | Service and parts
Revenues
Total revenues		520.3		1,019.2
Without Adoption of ASC 606 | Fleet and wholesale
Revenues
Total revenues		$ 331.4		$ 603.1