Revenue Information - Schedule of Revenue Categories (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue Information
Revenues (includes related party revenue of $3,164 and $8,184 for the three and nine months ended September 30, 2020, respectively, and related party contra revenue of $331 and $840 for the three and nine months ended September 30 and 2019, respectively)	$ 77,247	$ 85,785	$ 214,718	$ 250,162
Dealer revenue
Revenue Information
Revenues (includes related party revenue of $3,164 and $8,184 for the three and nine months ended September 30, 2020, respectively, and related party contra revenue of $331 and $840 for the three and nine months ended September 30 and 2019, respectively)	68,229	81,270	192,076	237,061
OEM incentives revenue
Revenue Information
Revenues (includes related party revenue of $3,164 and $8,184 for the three and nine months ended September 30, 2020, respectively, and related party contra revenue of $331 and $840 for the three and nine months ended September 30 and 2019, respectively)	5,815	4,383	14,109	12,727
Other revenue
Revenue Information
Revenues (includes related party revenue of $3,164 and $8,184 for the three and nine months ended September 30, 2020, respectively, and related party contra revenue of $331 and $840 for the three and nine months ended September 30 and 2019, respectively)	$ 3,203	$ 132	$ 8,533	$ 374