Segment Information (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information
Number of reportable segments | item	4
Revenues and segment income by reportable segment
Revenues	$ 5,564.4	$ 5,746.9
Segment income	$ 133.8	144.3
Retail Automotive Dealership
Segment Reporting Information
Number of reportable segments | item	1
Number of operating segments | item	6
Revenues and segment income by reportable segment
Revenues	$ 5,091.2	5,296.0	$ 20,800.0
Retail Automotive Dealership | Operating segments
Revenues and segment income by reportable segment
Revenues	5,091.2	5,296.0
Segment income	84.9	106.6
Retail Commercial Truck Dealership
Revenues and segment income by reportable segment
Revenues	332.3	292.4
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Revenues	332.3	292.4
Segment income	15.9	12.1
Other | Operating segments
Revenues and segment income by reportable segment
Revenues	140.9	158.5
Segment income	7.2	9.6
Non-Automotive Investments | Operating segments
Revenues and segment income by reportable segment
Segment income	$ 25.8	$ 16.0