Intangible Assets - Changes in Goodwill by Reportable Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of the changes in the carrying amount of goodwill by reportable segment
Goodwill, Beginning balance	$ 1,291.2	$ 1,323.1
Goodwill, Additions	324.8	24.9
Goodwill, Disposals	(4.5)	(1.7)
Goodwill, Foreign currency translation	49.0	(55.1)
Goodwill, Ending balance	1,660.5	1,291.2	$ 1,323.1
Impairment of intangible assets	0.0	0.0	0.0
Retail Automotive Dealership
Summary of the changes in the carrying amount of goodwill by reportable segment
Goodwill, Beginning balance	1,050.2	1,095.6
Goodwill, Additions	324.8	9.8
Goodwill, Disposals	(4.5)	(0.2)
Goodwill, Foreign currency translation	41.6	(55.0)
Goodwill, Ending balance	1,412.1	1,050.2	1,095.6
Retail Commercial Truck Dealership
Summary of the changes in the carrying amount of goodwill by reportable segment
Goodwill, Beginning balance	162.0	147.5
Goodwill, Additions		15.1
Goodwill, Foreign currency translation	1.0	(0.6)
Goodwill, Ending balance	163.0	162.0	147.5
Other
Summary of the changes in the carrying amount of goodwill by reportable segment
Goodwill, Beginning balance	79.0	80.0
Goodwill, Disposals		(1.5)
Goodwill, Foreign currency translation	6.4	0.5
Goodwill, Ending balance	85.4	79.0	$ 80.0
Non-Automotive Investments
Summary of the changes in the carrying amount of goodwill by reportable segment
Goodwill, Beginning balance	0.0
Goodwill, Ending balance	$ 0.0	$ 0.0