Revenues - Retail Commercial Truck 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
Retail Commercial Truck Dealership
Revenues
Total revenues									1,374.5	1,048.0	1,000.7
New vehicle | Retail Commercial Truck Dealership
Revenues
Total revenues									866.9
Used vehicle | Retail Commercial Truck Dealership
Revenues
Total revenues									112.0
Finance and insurance, net | Retail Commercial Truck Dealership
Revenues
Total revenues									11.9
Service and parts | Retail Commercial Truck Dealership
Revenues
Total revenues									364.5
Wholesale | Retail Commercial Truck Dealership
Revenues
Total revenues									$ 19.2
Without Adoption of ASC 606 | Retail Commercial Truck Dealership
Revenues
Total revenues										1,048.0	1,000.7
Without Adoption of ASC 606 | New vehicle | Retail Commercial Truck Dealership
Revenues
Total revenues										613.2	625.5
Without Adoption of ASC 606 | Used vehicle | Retail Commercial Truck Dealership
Revenues
Total revenues										89.4	51.8
Without Adoption of ASC 606 | Finance and insurance, net | Retail Commercial Truck Dealership
Revenues
Total revenues										8.9	7.3
Without Adoption of ASC 606 | Service and parts | Retail Commercial Truck Dealership
Revenues
Total revenues										325.6	306.0
Without Adoption of ASC 606 | Wholesale | Retail Commercial Truck Dealership
Revenues
Total revenues										$ 10.9	$ 10.1