Segment Information - Revenue by product type (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
Revenue from external customers by product type
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
Retail Automotive Dealership
Revenue from external customers by product type
Total revenues									20,849.2	19,824.3	18,673.2
Retail Automotive Dealership | New vehicle
Revenue from external customers by product type
Total revenues									9,666.4
Retail Automotive Dealership | Used vehicle
Revenue from external customers by product type
Total revenues									7,252.1
Retail Automotive Dealership | Finance and insurance, net
Revenue from external customers by product type
Total revenues									629.6
Retail Automotive Dealership | Service and parts
Revenue from external customers by product type
Total revenues									2,151.4
Retail Automotive Dealership | Fleet and wholesale
Revenue from external customers by product type
Total revenues									1,149.7
Retail Automotive Dealership | Operating segments
Revenue from external customers by product type
Total revenues									20,849.2	19,824.3	18,673.2
Retail Automotive Dealership | Operating segments | New vehicle
Revenue from external customers by product type
Total revenues									9,666.4	9,678.5	9,547.1
Retail Automotive Dealership | Operating segments | Used vehicle
Revenue from external customers by product type
Total revenues									7,252.1	6,386.8	5,663.7
Retail Automotive Dealership | Operating segments | Finance and insurance, net
Revenue from external customers by product type
Total revenues									629.6	581.8	495.0
Retail Automotive Dealership | Operating segments | Service and parts
Revenue from external customers by product type
Total revenues									2,151.4	2,057.5	1,948.6
Retail Automotive Dealership | Operating segments | Fleet and wholesale
Revenue from external customers by product type
Total revenues									1,149.7	1,119.7	1,018.8
Retail Commercial Truck Dealership
Revenue from external customers by product type
Total revenues									1,374.5	1,048.0	1,000.7
Retail Commercial Truck Dealership | New vehicle
Revenue from external customers by product type
Total revenues									866.9
Retail Commercial Truck Dealership | Used vehicle
Revenue from external customers by product type
Total revenues									112.0
Retail Commercial Truck Dealership | Finance and insurance, net
Revenue from external customers by product type
Total revenues									11.9
Retail Commercial Truck Dealership | Service and parts
Revenue from external customers by product type
Total revenues									364.5
Retail Commercial Truck Dealership | Operating segments
Revenue from external customers by product type
Total revenues									1,374.5	1,048.0	1,000.7
Retail Commercial Truck Dealership | Operating segments | New vehicle
Revenue from external customers by product type
Total revenues									866.9	613.2	625.5
Retail Commercial Truck Dealership | Operating segments | Used vehicle
Revenue from external customers by product type
Total revenues									112.0	89.4	51.8
Retail Commercial Truck Dealership | Operating segments | Finance and insurance, net
Revenue from external customers by product type
Total revenues									11.9	9.0	7.3
Retail Commercial Truck Dealership | Operating segments | Service and parts
Revenue from external customers by product type
Total revenues									364.5	325.6	306.0
Retail Commercial Truck Dealership | Operating segments | Lease, rental and wholesale
Revenue from external customers by product type
Total revenues									$ 19.2	$ 10.8	$ 10.1