Loans Held for Investment - By Region (Details)	Dec. 31, 2017	Dec. 31, 2016
Loans held for investment by Region [Line Items]
Percentage of Loans concentrated in major states	40.40%	40.30%
New York
Loans held for investment by Region [Line Items]
Percentage of Loans held in state	10.50%	10.30%
California
Loans held for investment by Region [Line Items]
Percentage of Loans held in state	9.30%	9.40%
Pennsylvania
Loans held for investment by Region [Line Items]
Percentage of Loans held in state	8.50%	8.40%
New Jersey
Loans held for investment by Region [Line Items]
Percentage of Loans held in state	6.90%	6.90%
Illinois
Loans held for investment by Region [Line Items]
Percentage of Loans held in state	5.20%	5.30%