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Segment Information (Details) $ in Millions	3 Months Ended		12 Months Ended
Segment Information (Details) $ in Millions	Mar. 31, 2021 USD ($) item	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information
Number of reportable segments \| item	4
Revenues and segment income by reportable segment
Revenues	$ 5,773.8	$ 5,009.1
Segment income	$ 247.6	71.5
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	$ 5,206.9	4,416.6	$ 17,900.0
Retail Automotive Dealership \| Operating segments
Revenues and segment income by reportable segment
Revenues	5,206.9	4,416.6
Segment income	160.3	41.3
Retail Commercial Truck Dealership
Revenues and segment income by reportable segment
Revenues	434.7	491.4
Retail Commercial Truck Dealership \| Operating segments
Revenues and segment income by reportable segment
Revenues	434.7	491.4
Segment income	27.5	13.7
Other \| Operating segments
Revenues and segment income by reportable segment
Revenues	132.2	101.1
Segment income	6.0	2.9
Non-Automotive Investments \| Operating segments
Revenues and segment income by reportable segment
Segment income	$ 53.8	$ 13.6