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Description of Business (Percentage of Consumer Loans Assigned Based on Volumes) (Details) | 3 Months Ended
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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.

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