Related Party Transactions - Shared Services Agreement with DriveTime (Details) - Affiliated Entity - DriveTime Automotive Group, Inc. - Shared Services Agreement with DriveTime - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Related Party Transaction [Line Items]
Number of allowable days prior to contract termination with written notice	30 days
Number of allowable days prior to contract termination with written notice from service provider	90 days
Selling, general and administrative
Related Party Transaction [Line Items]
Expenses from transactions with related party		$ 0.0	$ 0.2	$ 0.1	$ 0.4