Loans Held for Investment and Allowance for Loan Losses (Aging Analysis of Term Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current loans	$ 1,002,932	$ 850,060
Total unpaid principal balance	1,095,792	936,239
1-14 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	22,573	23,611
15-29 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	10,261	12,528
30-59 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	18,671	22,059
60-89 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	15,161	12,809
90 calendar days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Past due loans	$ 26,194	$ 15,172