Receivables and Contract Assets, Net - Financial Assets (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contracts-in-transit	$ 142.5	$ 169.9
Vehicle receivables	76.7	84.3
Total contracts-in-transit and vehicle receivables	219.2	254.1
Less: allowance for doubtful accounts	0.3	0.3
Total contracts-in-transit and vehicle receivables, net	218.9	253.8
Accounts and notes receivable
Accounts and notes receivables, net:	197.1	227.9
Less: allowance for doubtful accounts	3.6	2.8
Total accounts and notes receivables, net	193.5	225.1
Total contract assets, net	29.7	21.6
Manufacturer receivables
Accounts and notes receivable
Accounts and notes receivables, net:	108.5	123.9
Parts and service receivables
Accounts and notes receivable
Accounts and notes receivables, net:	51.9	57.0
F&I receivables
Accounts and notes receivable
Accounts and notes receivables, net:	24.9	28.3
Other
Accounts and notes receivable
Accounts and notes receivables, net:	$ 11.8	$ 18.7