Basis and Summary of Significant Accounting Policies - Credit Quality Indicators (Details) |
Sep. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|
| FICO Score, 600 or Less | ||
| Financing Receivable, Recorded Investment [Line Items] | ||
| Financing receivable, percentage of loan portfolio per FICO Score | 3.10% | 1.70% |
| FICO Score, Between 600 and 700 | ||
| Financing Receivable, Recorded Investment [Line Items] | ||
| Financing receivable, percentage of loan portfolio per FICO Score | 77.50% | 76.50% |
| FICO Score, 700 or Greater | ||
| Financing Receivable, Recorded Investment [Line Items] | ||
| Financing receivable, percentage of loan portfolio per FICO Score | 19.40% | 21.80% |