Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) financial_institution state segment store brand finance_companies	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of stores | store	187
Number of new vehicle brands | brand	30
Number of states | state	19
Debt Instrument [Line Items]
Threshold period for interest to bear on receivables	60 days
Interest costs capitalized	$ 2.3	$ 1.3	$ 0.5
Accrued warranty balance	0.6	0.5
Contract liability	171.5	149.6
Contract liability, revenue recognized	25.9	21.9
Contract asset	$ 8.9	$ 9.2
Number of reportable segments | segment	3
Minimum
Debt Instrument [Line Items]
Period that contracts in transit are outstanding	5 days
Maximum
Debt Instrument [Line Items]
Period that contracts in transit are outstanding	10 days
Credit Facility | Line of Credit
Debt Instrument [Line Items]
Number of financial institutions | financial_institution	19
Number of manufacturer affiliated finance companies | finance_companies	7