INVENTORIES (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Components of Inventory [Line Items]
Total inventories	$ 962.8		$ 821.9	$ 826.0
Lower of cost or market inventory reserves	6.2			5.7
New vehicles
Components of Inventory [Line Items]
Total inventories	772.6			646.5
Reduction of new vehicle inventory cost by automobile manufacturer incentives	(8.7)			(7.4)
Reduction to cost of sales	30.6	$ 29.4
Used vehicles
Components of Inventory [Line Items]
Total inventories	149.9			135.9
Parts and accessories
Components of Inventory [Line Items]
Total inventories	$ 40.3			$ 43.6