Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Summary of Significant Accounting Policies - Restricted Cash (Details)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Floor Plan Facility \| Line of Credit
Debt Instrument [Line Items]
Deposit required under floor plan facility, percentage of principal balance	5.00%	5.00%	5.00%	5.00%