Accounts Receivable and Contract Assets (Summary of Accounts Receivable) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 584.3		$ 567.4
Less: Allowance for doubtful accounts	(6.3)		(7.3)
Less: Long-term portion of accounts receivable, net	(66.3)		(55.1)
Accounts receivable, net	511.7	$ 504.5	505.0
Contracts in transit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	235.9		269.7
Trade receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	56.7		52.8
Vehicle receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	55.3		50.9
Manufacturer receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	107.1		112.4
Auto loan receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	95.8		62.2
Less: Allowance for doubtful accounts	(13.9)
Other receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable	$ 33.5		$ 19.4