Basis and Summary of Significant Accounting Policies - Credit Quality Indicators (Details)	Mar. 31, 2019	Dec. 31, 2018
FICO Score, 600 or Less
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	4.10%	3.70%
FICO Score, Between 600 and 700
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	79.00%	77.90%
FICO Score, 700 or Greater
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	16.90%	18.40%