Business Combinations - Activity (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 item
Business combinations
Revenue from acquisition | $	$ 620.2
Pre-tax income from acquisition | $	$ 24.1
The Car People
Business combinations
Number of retail locations acquired		4
Retail Automotive Dealership
Business combinations
Number of retail locations acquired		6
Retail Commercial Truck Dealership
Business combinations
Number of retail locations acquired	6	1
Commercial Vehicle Distribution
Business combinations
Number of dealerships acquired	1