Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information
Number of Reportable Segments | item									3
Revenues and segment income by reportable segment
Total revenues	$ 4,921.3	$ 4,960.1	$ 4,920.6	$ 4,482.9	$ 4,425.6	$ 4,396.7	$ 4,384.5	$ 4,025.2	$ 19,284.9	$ 17,232.0	$ 14,482.5
Floor plan interest expense									44.5	46.5	43.4
Other interest expense									69.4	52.8	45.3
Depreciation									78.0	70.2	59.6
Equity in earnings of affiliates									39.3	40.8	30.7
Adjusted Segment Income									491.9	441.9	371.8
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									491.9	441.9	371.8
Gain on investment										16.0
Income from continuing operations before income taxes									491.9	457.9	371.8
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	8,022.7				7,228.2				8,022.7	7,228.2
Equity method investments	336.4				352.8				336.4	352.8
Capital expenditures									199.5	176.1	175.0
Intersegment Elimination
Revenues and segment income by reportable segment
Total revenues									$ (1.9)	(5.1)
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets					(0.4)					(0.4)
Retail Automotive
Segment Reporting Information
Number of geographic operating segments | item									4
Number of geographic reporting units | item									1
Retail Automotive | Operating segments
Revenues and segment income by reportable segment
Total revenues									$ 17,896.3	16,657.5	14,329.9
Floor plan interest expense									41.3	45.1	42.8
Other interest expense									56.8	46.9	44.2
Depreciation									71.3	67.1	59.1
Equity in earnings of affiliates									4.4	3.8	4.9
Adjusted Segment Income									420.4	390.1	338.3
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									420.4	390.1	338.3
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	6,691.4				5,920.4				6,691.4	5,920.4
Equity method investments	38.3				62.8				38.3	62.8
Capital expenditures									187.7	170.8	175.0
Retail Commercial Truck Dealership | Operating segments
Revenues and segment income by reportable segment
Total revenues									944.1	125.6
Floor plan interest expense									2.0	0.3
Other interest expense									3.8	0.9
Depreciation									2.0	0.5
Equity in earnings of affiliates										6.5	1.2
Adjusted Segment Income									35.4	11.7	1.2
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									35.4	11.7	1.2
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	525.5				366.1				525.5	366.1
Capital expenditures									4.5	0.4
Other | Operating segments
Revenues and segment income by reportable segment
Total revenues									446.4	454.0	152.6
Floor plan interest expense									1.2	1.1	0.6
Other interest expense									8.8	5.0	1.1
Depreciation									4.7	2.6	0.5
Equity in earnings of affiliates									34.9	30.5	24.6
Adjusted Segment Income									36.1	40.1	32.3
Reconciliation of total adjusted segment income to consolidated income from continuing operations before income taxes
Adjusted Segment Income									36.1	40.1	$ 32.3
Total assets, equity method investments, and capital expenditures by reporting segment
Total assets	805.8				942.1				805.8	942.1
Equity method investments	$ 298.1				$ 290.0				298.1	290.0
Capital expenditures									$ 7.3	$ 4.9