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Segment Information (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
Segment Information (Details) $ in Millions	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) item	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information
Number of reportable segments \| item			4
Number of operating segments \| item			6
Revenues and segment income by reportable segment
Revenues	$ 6,987.5	$ 3,651.1	$ 12,761.3	$ 8,660.2
Segment income	463.7	61.2	$ 711.3	132.7
Operating segments
Segment Reporting Information
Number of reportable segments \| item			1
Retail automotive dealership
Revenues and segment income by reportable segment
Revenues	6,197.6	3,153.5	$ 11,404.5	7,570.1	$ 17,900.0
Retail automotive dealership \| Operating segments
Revenues and segment income by reportable segment
Revenues	6,197.6	3,153.5	11,404.5	7,570.1
Segment income	313.0	11.8	473.3	53.1
Retail commercial truck dealership
Revenues and segment income by reportable segment
Revenues	625.3	399.2	1,060.0	890.6
Retail commercial truck dealership \| Operating segments
Revenues and segment income by reportable segment
Revenues	625.3	399.2	1,060.0	890.6
Segment income	39.7	14.6	67.2	28.3
Other \| Operating segments
Revenues and segment income by reportable segment
Revenues	164.6	98.4	296.8	199.5
Segment income	8.4	4.9	14.4	7.8
Non-Automotive Investments \| Operating segments
Revenues and segment income by reportable segment
Segment income	$ 102.6	$ 29.9	$ 156.4	$ 43.5