Subsequent Events - Narrative (Details)			9 Months Ended
	Nov. 06, 2018 USD ($) shares	Nov. 02, 2018 USD ($) multiplier	Sep. 30, 2018	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Floor Plan Facility | Line of Credit
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity (up to)				$ 350,000,000.0	$ 275,000,000.0
Floor Plan Facility | Line of Credit | London Interbank Offered Rate (LIBOR)
Subsequent Event [Line Items]
Debt instrument, basis spread on variable rate			3.65%
Subsequent Event | Class A | Contribution Agreement | Chief Executive Officer
Subsequent Event [Line Items]
Stock contribution commitment (in shares) | shares	32,932
Contribution of Class A common stock from related party, fee charged	$ 0
Subsequent Event | Floor Plan Facility | Line of Credit
Subsequent Event [Line Items]
Line of credit facility, maximum borrowing capacity (up to)		$ 650,000,000.0
Subsequent Event | Floor Plan Facility | Line of Credit | London Interbank Offered Rate (LIBOR)
Subsequent Event [Line Items]
Debt instrument, basis spread on variable rate		3.40%
Subsequent Event | Consumer Loan | Purchase and Sale Agreement
Subsequent Event [Line Items]
Commitment of purchaser, current availability financing, principal balances of finance receivables (up to)		$ 1,250,000,000
Subsequent Event | Consumer Loan | Master Transfer Agreement
Subsequent Event [Line Items]
Commitment of purchaser, current availability financing, principal balances of finance receivables (up to)		$ 454,500,000
Purchase commitment multiplier | multiplier		3