Loans Held for Investment, Allowance for Loan Losses and Loans Held for Sale (Aging Analysis of Term Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Non-delinquent loans	$ 850,060	$ 890,297
Total unpaid principal balance	936,239	980,451
1-14 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	23,611	25,899
15-29 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	12,528	15,990
30-59 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	22,059	22,677
60-89 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	12,809	13,952
90 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	$ 15,172	$ 11,636