Segment Information - Reporting information by segment (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information
Number of reportable segments | item									4
Revenues and segment income by reportable segment
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
Floor plan interest expense									80.9	63.4	50.9
Other interest expense									114.7	107.4	85.4
Depreciation									103.7	95.1	89.7
Equity in earnings of affiliates									134.8	107.6	69.5
Adjusted segment income									604.1	548.2	508.1
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	10,904.5				10,540.6				10,904.5	10,540.6
Equity method investments	1,305.2				1,256.6				1,305.2	1,256.6
Capital expenditures									$ 305.6	247.0	203.1
Intersegment Elimination
Revenues and segment income by reportable segment
Total revenues											(3.9)
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									$ 20,849.2	19,824.3	18,673.2
Retail Automotive Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									20,849.2	19,824.3	18,673.2
Floor plan interest expense									74.9	59.4	47.1
Other interest expense									108.3	95.0	74.8
Depreciation									94.2	85.7	81.7
Equity in earnings of affiliates									5.2	4.6	4.9
Adjusted segment income									389.7	397.2	412.9
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	8,501.4				8,157.0				8,501.4	8,157.0
Equity method investments	64.1				68.5				64.1	68.5
Capital expenditures									292.6	237.8	195.6
Retail Commercial Truck Dealership
Revenues and segment income by reportable segment
Total revenues									1,374.5	1,048.0	1,000.7
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									1,374.5	1,048.0	1,000.7
Floor plan interest expense									4.2	2.7	2.9
Other interest expense									2.4	3.4	4.1
Depreciation									4.3	4.1	3.2
Adjusted segment income									62.3	38.4	26.2
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	571.5				553.4				571.5	553.4
Capital expenditures									9.3	6.4	3.3
Other | Operating segments
Revenues and segment income by reportable segment
Total revenues									561.4	514.6	448.5
Floor plan interest expense									1.8	1.3	0.9
Other interest expense									4.0	9.0	6.5
Depreciation									5.2	5.3	4.8
Adjusted segment income									22.5	9.6	4.4
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	590.5				642.1				590.5	642.1
Capital expenditures									3.7	2.8	4.2
Non-Automotive Investments | Operating segments
Revenues and segment income by reportable segment
Equity in earnings of affiliates									129.6	103.0	64.6
Adjusted segment income									129.6	103.0	$ 64.6
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	1,241.1				1,188.1				1,241.1	1,188.1
Equity method investments	$ 1,241.1				$ 1,188.1				$ 1,241.1	$ 1,188.1