Revenues - Retail Automotive Dealership (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Total revenues	$ 5,439.3	$ 5,658.6	$ 5,940.3	$ 5,746.9	$ 5,398.0	$ 5,524.4	$ 5,383.4	$ 5,081.1	$ 22,785.1	$ 21,386.9	$ 20,118.5
U.S.
Revenues
Total revenues									12,607.8	12,487.2	12,005.9
Retail Automotive Dealership
Revenues
Total revenues									20,849.2	19,824.3	18,673.2
Retail Automotive Dealership | U.S.
Revenues
Total revenues									11,504.3
Retail Automotive Dealership | UK
Revenues
Total revenues									7,961.4
Retail Automotive Dealership | Germany and Italy
Revenues
Total revenues									1,383.5
New vehicle | Retail Automotive Dealership
Revenues
Total revenues									9,666.4
Used vehicle | Retail Automotive Dealership
Revenues
Total revenues									7,252.1
Finance and insurance, net | Retail Automotive Dealership
Revenues
Total revenues									629.6
Service and parts | Retail Automotive Dealership
Revenues
Total revenues									2,151.4
Fleet and wholesale | Retail Automotive Dealership
Revenues
Total revenues									$ 1,149.7
Without Adoption of ASC 606 | Retail Automotive Dealership
Revenues
Total revenues										19,824.3	18,673.2
Without Adoption of ASC 606 | Retail Automotive Dealership | U.S.
Revenues
Total revenues										11,610.1	11,109.6
Without Adoption of ASC 606 | Retail Automotive Dealership | UK
Revenues
Total revenues										7,048.7	6,520.1
Without Adoption of ASC 606 | Retail Automotive Dealership | Germany and Italy
Revenues
Total revenues										1,165.5	1,043.5
Without Adoption of ASC 606 | New vehicle | Retail Automotive Dealership
Revenues
Total revenues										9,678.5	9,547.1
Without Adoption of ASC 606 | Used vehicle | Retail Automotive Dealership
Revenues
Total revenues										6,386.8	5,663.7
Without Adoption of ASC 606 | Finance and insurance, net | Retail Automotive Dealership
Revenues
Total revenues										581.8	495.0
Without Adoption of ASC 606 | Service and parts | Retail Automotive Dealership
Revenues
Total revenues										2,057.5	1,948.6
Without Adoption of ASC 606 | Fleet and wholesale | Retail Automotive Dealership
Revenues
Total revenues										$ 1,119.7	$ 1,018.8