Floorplan Notes Payable - Schedule of Floorplan Notes Payable (Details) - Line of credit - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Revolving credit facility — floorplan notes payable
Line of Credit Facility [Line Items]
Long-term debt, gross	$ 840.4	$ 1,206.0
Long-term debt, offset	(108.2)	(106.8)
Long-term debt	732.2	1,099.1
Other non-manufacturer facilities
Line of Credit Facility [Line Items]
Long-term debt	39.0	45.3
Floorplan notes payable — credit facility and other, net
Line of Credit Facility [Line Items]
Long-term debt	771.3	1,144.4
FMCC facility
Line of Credit Facility [Line Items]
Long-term debt, gross	133.2	208.5
Long-term debt, offset	(18.5)	(4.1)
Long-term debt	114.7	204.5
Other manufacturer affiliate facilities
Line of Credit Facility [Line Items]
Long-term debt	200.1	255.4
Floorplan notes payable — manufacturer affiliates, net
Line of Credit Facility [Line Items]
Long-term debt	$ 314.8	$ 459.9