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%