Segment Information - Reporting information by segment (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information
Number of reportable segments | item									4
Revenues and segment income by reportable segment
Total revenues	$ 5,398.0	$ 5,524.4	$ 5,383.4	$ 5,081.1	$ 4,887.8	$ 5,152.0	$ 5,254.1	$ 4,824.6	$ 21,386.9	$ 20,118.5	$ 19,284.9
Floor plan interest expense									63.4	50.9	44.5
Other interest expense									107.4	85.4	69.4
Depreciation									95.1	89.7	78.0
Equity in earnings of affiliates									107.6	69.5	39.3
Adjusted segment income									548.2	508.1	491.9
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	10,540.6				8,833.0				10,540.6	8,833.0
Equity method investments	1,256.6				893.4				1,256.6	893.4
Capital expenditures									$ 247.0	203.1	199.5
Intersegment Elimination
Revenues and segment income by reportable segment
Total revenues										(3.9)	(1.9)
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									$ 19,824.3	18,673.2	17,896.3
Retail Automotive Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									19,824.3	18,673.2	17,896.3
Floor plan interest expense									59.4	47.1	41.3
Other interest expense									95.0	74.8	56.8
Depreciation									85.7	81.7	71.3
Equity in earnings of affiliates									4.6	4.9	4.4
Adjusted segment income									397.2	412.9	420.4
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	8,157.0				7,011.5				8,157.0	7,011.5
Equity method investments	68.5				58.6				68.5	58.6
Capital expenditures									237.8	195.6	187.7
Retail Commercial Truck Dealership
Revenues and segment income by reportable segment
Total revenues									1,048.0	1,000.7	944.1
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									1,048.0	1,000.7	944.1
Floor plan interest expense									2.7	2.9	2.0
Other interest expense									3.4	4.1	3.8
Depreciation									4.1	3.2	2.0
Adjusted segment income									38.4	26.2	35.4
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	553.4				489.1				553.4	489.1
Capital expenditures									6.4	3.3	4.5
Other | Operating segments
Revenues and segment income by reportable segment
Total revenues									514.6	448.5	446.4
Floor plan interest expense									1.3	0.9	1.2
Other interest expense									9.0	6.5	8.8
Depreciation									5.3	4.8	4.7
Adjusted segment income									9.6	4.4	1.2
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	642.1				497.6				642.1	497.6
Capital expenditures									2.8	4.2	7.3
Non-Automotive Investments | Operating segments
Revenues and segment income by reportable segment
Equity in earnings of affiliates									103.0	64.6	34.9
Adjusted segment income									103.0	64.6	$ 34.9
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	1,188.1				834.8				1,188.1	834.8
Equity method investments	$ 1,188.1				$ 834.8				$ 1,188.1	$ 834.8