Description of Business (Percentage of Consumer Loans Assigned Based on Volumes) (Details)		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Portfolio Program [Member]
Description Of Business [Line Items]
Percentage of new consumer loans unit volume		70.10%	72.50%	71.90%	72.30%	73.30%
Percentage of new consumer loans dollar volume	[1]	67.40%	69.70%	68.60%	67.90%	67.80%
Purchase Program [Member]
Description Of Business [Line Items]
Percentage of new consumer loans unit volume		29.90%	27.50%	28.10%	27.70%	26.70%
Percentage of new consumer loans dollar volume	[1]	32.60%	30.30%	31.40%	32.10%	32.20%

[1] Represents advances paid to Dealers on Consumer Loans assigned under our Portfolio Program and one-time payments made to Dealers to purchase Consumer Loans assigned under our Purchase Program. Payments of Dealer Holdback and accelerated Dealer Holdback are not included.