Basis and Summary of Significant Accounting Policies - Credit Quality Indicators (Details)	Mar. 31, 2018	Dec. 31, 2017
FICO Score, 600 or Less
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	2.00%	1.70%
FICO Score, Between 600 and 700
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	77.00%	76.50%
FICO Score, 700 or Greater
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	21.00%	21.80%