Acquisitions - Pro Forma Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Assets Or Stock Of Multiple Dealership Locations Acquired
Acquisitions
Revenue	$ 300.8	$ 75.1
Pre-tax income (loss)	14.2	$ 2.7
Gander Mountain and Overton's
Acquisitions
Revenue	27.5
Pre-tax income (loss)	33.8
TheHouse.com
Acquisitions
Revenue	32.7
Pre-tax income (loss)	4.7
W82 & Uncle Dan's
Acquisitions
Revenue	4.3
Pre-tax income (loss)	$ 0.3