Debt Instruments - Past Finance Receivable Facilities (Details) - Revolving Credit Facility - Ally Bank - USD ($)	1 Months Ended
	May 31, 2019	Apr. 30, 2019
Loan and Security Agreement | DART I Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 300,000,000
Deposit required under floor plan facility, percentage of principal balance	2.00%
Loan and Security Agreement | DART I Credit Facility | Minimum | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.00%
Loan and Security Agreement | DART I Credit Facility | Maximum | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.80%
Amended and Restated Loan And Security Agreement | SART 2017-1 Credit Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 350,000,000
Deposit required under floor plan facility, percentage of principal balance	2.00%
Amended and Restated Loan And Security Agreement | SART 2017-1 Credit Facility | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.95%