Loans Held for Investment and Allowance for Loan Losses (Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unpaid principal balance	$ 1,095,792	$ 936,239
Net deferred origination costs	22,036	16,557
Total loans held for investment	1,117,828	952,796
Term loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unpaid principal balance	928,091	804,227
Lines of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total unpaid principal balance	$ 167,702	$ 132,012