Segment Information - Revenue by product type (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from external customers by product type
Total revenues	$ 5,398.0	$ 5,524.4	$ 5,383.4	$ 5,081.1	$ 4,887.8	$ 5,152.0	$ 5,254.1	$ 4,824.6	$ 21,386.9	$ 20,118.5	$ 19,284.9
Retail Automotive Dealership
Revenue from external customers by product type
Total revenues									19,824.3	18,673.2	17,896.3
Retail Automotive Dealership | Operating segments
Revenue from external customers by product type
Total revenues									19,824.3	18,673.2	17,896.3
Retail Automotive Dealership | Operating segments | New vehicle
Revenue from external customers by product type
Total revenues									9,678.5	9,547.1	9,208.9
Retail Automotive Dealership | Operating segments | Used vehicle
Revenue from external customers by product type
Total revenues									6,386.8	5,663.7	5,425.5
Retail Automotive Dealership | Operating segments | Finance and insurance, net
Revenue from external customers by product type
Total revenues									581.8	495.0	478.3
Retail Automotive Dealership | Operating segments | Service and parts
Revenue from external customers by product type
Total revenues									2,057.5	1,948.6	1,830.7
Retail Automotive Dealership | Operating segments | Fleet and wholesale
Revenue from external customers by product type
Total revenues									1,119.7	1,018.8	952.9
Retail Commercial Truck Dealership
Revenue from external customers by product type
Total revenues									1,048.0	1,000.7	944.1
Retail Commercial Truck Dealership | Operating segments
Revenue from external customers by product type
Total revenues									1,048.0	1,000.7	944.1
Retail Commercial Truck Dealership | Operating segments | New vehicle
Revenue from external customers by product type
Total revenues									613.2	625.5	572.5
Retail Commercial Truck Dealership | Operating segments | Used vehicle
Revenue from external customers by product type
Total revenues									89.4	51.8	58.7
Retail Commercial Truck Dealership | Operating segments | Finance and insurance, net
Revenue from external customers by product type
Total revenues									9.0	7.3	6.7
Retail Commercial Truck Dealership | Operating segments | Service and parts
Revenue from external customers by product type
Total revenues									325.6	306.0	286.0
Retail Commercial Truck Dealership | Operating segments | Lease, rental and wholesale
Revenue from external customers by product type
Total revenues									$ 10.8	$ 10.1	$ 20.2