Segment Information - Reporting information by segment (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information
Number of Reportable Segments | item									4
Revenues and segment income by reportable segment
Total revenues	$ 4,887.8	$ 5,152.0	$ 5,254.1	$ 4,824.6	$ 4,921.3	$ 4,960.1	$ 4,920.6	$ 4,482.9	$ 20,118.5	$ 19,284.9	$ 17,232.0
Floor plan interest expense									50.9	44.5	46.5
Other interest expense									85.4	69.4	52.8
Depreciation									89.7	78.0	70.2
Equity in earnings of affiliates									69.5	39.3	40.8
Adjusted Segment Income									508.1	491.9	441.9
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									508.1	491.9	441.9
Gain on investment											16.0
Income from continuing operations before income taxes									508.1	491.9	457.9
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	8,861.1				8,013.4				8,861.1	8,013.4
Equity method investments	893.4				336.4				893.4	336.4
Capital expenditures									203.1	199.5	176.1
Intersegment Elimination
Revenues and segment income by reportable segment
Total revenues									$ (3.9)	(1.9)	(5.1)
Retail Automotive Dealership
Segment Reporting Information
Number of geographic operating segments | item									4
Number of geographic reporting units | item									1
Revenues and segment income by reportable segment
Total revenues									$ 18,673.2	17,896.3	16,657.5
Retail Automotive Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									18,673.2	17,896.3	16,657.5
Floor plan interest expense									47.1	41.3	45.1
Other interest expense									74.8	56.8	46.9
Depreciation									81.7	71.3	67.1
Equity in earnings of affiliates									4.9	4.4	3.8
Adjusted Segment Income									412.9	420.4	390.1
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									412.9	420.4	390.1
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	7,039.6				6,682.1				7,039.6	6,682.1
Equity method investments	58.6				38.3				58.6	38.3
Capital expenditures									195.6	187.7	170.8
Retail Commercial Truck Dealership
Revenues and segment income by reportable segment
Total revenues									1,000.7	944.1	125.6
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									1,000.7	944.1	125.6
Floor plan interest expense									2.9	2.0	0.3
Other interest expense									4.1	3.8	0.9
Depreciation									3.2	2.0	0.5
Equity in earnings of affiliates											6.5
Adjusted Segment Income									26.2	35.4	11.7
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									26.2	35.4	11.7
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	489.1				525.5				489.1	525.5
Capital expenditures									3.3	4.5	0.4
Other | Operating segments
Revenues and segment income by reportable segment
Total revenues									448.5	446.4	454.0
Floor plan interest expense									0.9	1.2	1.1
Other interest expense									6.5	8.8	5.0
Depreciation									4.8	4.7	2.6
Adjusted Segment Income									4.4	1.2	9.6
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									4.4	1.2	9.6
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	497.6				507.7				497.6	507.7
Capital expenditures									4.2	7.3	4.9
Non-Automotive Investments | Operating segments
Revenues and segment income by reportable segment
Equity in earnings of affiliates									64.6	34.9	30.5
Adjusted Segment Income									64.6	34.9	30.5
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									64.6	34.9	$ 30.5
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	834.8				298.1				834.8	298.1
Equity method investments	$ 834.8				$ 298.1				$ 834.8	$ 298.1