Related Party Transactions - Aircraft Time Sharing Agreement (Details) - Affiliated Entity - DriveTime Automotive Group, Inc. - Aircraft Time Sharing Agreement - Air Transportation Equipment $ in Millions		3 Months Ended		9 Months Ended
	Oct. 22, 2015 aircraft	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Related Party Transaction [Line Items]
Number of aircrafts | aircraft	2
Contractual agreement term	12 months
Contractual agreement, perpetual automatic renewal termPerpetual Automatic Renewal Term	12 months
Number of allowable days prior to contract termination with written notice	30 days
Expenses from transactions with related party | $		$ 0.2	$ 0.0	$ 0.4	$ 0.4