Basis and Summary of Significant Accounting Policies - Credit Quality Indicators (Details)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, threshold of past dues financing receivable that requires allowance	24 months
FICO Score, 600 or Less
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	1.70%	1.80%
FICO Score, Between 600 and 700
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	76.90%	78.10%
FICO Score, 700 or Greater
Financing Receivable, Recorded Investment [Line Items]
Financing receivable, percentage of loan portfolio per FICO Score	21.40%	20.10%