Credit Facilities and Long-term Debt - Schedule of Credit Facilities and Long-term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 09, 2019	Jun. 30, 2019	Dec. 31, 2018	Jul. 24, 2017
Debt Instrument [Line Items]
Floor plan notes payable: non-trade	$ 1,642.4		$ 1,600.0	$ 1,733.3
Floor plan notes payable	425.2			324.4
Total floor plan debt	2,067.6			2,057.7
Total long-term debt outstanding	1,480.3			1,390.1
Less: unamortized debt issuance costs	(10.4)			(6.0)
Less: current maturities (net of current debt issuance costs)	(39.3)			(25.9)
Long-term debt, less current maturities	1,430.6			1,358.2
Line of Credit | Used vehicle inventory financing facility
Debt Instrument [Line Items]
Total long-term debt outstanding	149.0			332.0
Line of Credit | Revolving lines of credit
Debt Instrument [Line Items]
Total long-term debt outstanding	0.0			131.6
Real estate mortgages
Debt Instrument [Line Items]
Total long-term debt outstanding	597.7			592.3
Other debt
Debt Instrument [Line Items]
Total long-term debt outstanding	33.6			34.2
5.250% Senior notes due 2025 | Senior Notes
Debt Instrument [Line Items]
Total long-term debt outstanding	$ 300.0			300.0
Interest rate, stated percentage	5.25%				5.25%
4.625% Senior notes due 2027 | Senior Notes
Debt Instrument [Line Items]
Total long-term debt outstanding	$ 400.0			$ 0.0
Interest rate, stated percentage	4.625%	4.625%