Business and Summary of Significant Accounting Policies - Credit Quality Indicators (Details) |
Dec. 31, 2017 |
Dec. 31, 2016 |
|---|---|---|
| FICO Score, Less than 600 | ||
| Financing Receivable, Recorded Investment [Line Items] | ||
| Financing Receivable, Percentage of Loan Portfolio per FICO Score | 1.70% | 1.80% |
| FICO Score, 600 to 699 | ||
| Financing Receivable, Recorded Investment [Line Items] | ||
| Financing Receivable, Percentage of Loan Portfolio per FICO Score | 76.50% | 78.10% |
| FICO Score, Greater than 700 | ||
| Financing Receivable, Recorded Investment [Line Items] | ||
| Financing Receivable, Percentage of Loan Portfolio per FICO Score | 21.80% | 20.10% |