Business Combinations - Activity (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 item
Business combinations
Revenue from acquisition | $	$ 1,046.4
Pre-tax income from acquisition | $	$ 37.1
CarSense
Business combinations
Number of retail locations acquired	5
CarShop
Business combinations
Number of retail locations acquired	5
Retail Automotive Dealership
Business combinations
Number of acquired franchises	8	20
Retail Commercial Truck Dealership
Business combinations
Number of retail locations acquired		7