Segment Information - Reporting information by segment (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information
Number of reportable segments | item	4
Revenues and segment income by reportable segment
Total revenues	$ 23,179.4	$ 22,785.1	$ 21,386.9
Floor plan interest expense	84.5	80.9	63.4
Other interest expense	124.2	114.7	107.4
Depreciation	109.6	103.7	95.1
Equity in earnings of affiliates	147.5	134.8	107.6
Segment income	591.5	604.1	548.2
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	13,942.7	10,904.5
Equity method investments	1,399.0	1,305.2
Capital expenditures	$ 245.3	305.6	247.0
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
Total revenues	$ 20,615.8	20,849.2	19,824.3
Retail Automotive Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues	20,615.8	20,849.2	19,824.3
Floor plan interest expense	74.9	74.9	59.4
Other interest expense	118.4	108.3	95.0
Depreciation	99.1	94.2	85.7
Equity in earnings of affiliates	5.2	5.2	4.6
Segment income	339.9	389.7	397.2
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	10,960.1	8,501.4
Equity method investments	72.1	64.1
Capital expenditures	231.9	292.6	237.8
Retail Commercial Truck Dealership
Revenues and segment income by reportable segment
Total revenues	2,050.5	1,374.5	1,048.0
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues	2,050.5	1,374.5	1,048.0
Floor plan interest expense	8.0	4.2	2.7
Other interest expense	3.2	2.4	3.4
Depreciation	5.5	4.3	4.1
Segment income	86.5	62.3	38.4
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	1,075.8	571.5
Capital expenditures	9.9	9.3	6.4
Other | Operating segments
Revenues and segment income by reportable segment
Total revenues	513.1	561.4	514.6
Floor plan interest expense	1.6	1.8	1.3
Other interest expense	2.6	4.0	9.0
Depreciation	5.0	5.2	5.3
Segment income	22.8	22.5	9.6
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	579.9	590.5
Capital expenditures	3.5	3.7	2.8
Non-Automotive Investments | Operating segments
Revenues and segment income by reportable segment
Equity in earnings of affiliates	142.3	129.6	103.0
Segment income	142.3	129.6	$ 103.0
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	1,326.9	1,241.1
Equity method investments	$ 1,326.9	$ 1,241.1