Loans Held for Investment - Summary of Loan Portfolio Activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Loans And Leases Receivable Disclosure [Abstract]
Balance at December 31, 2017	$ 3,175,672
Loans originated	1,622,084
Additional fundings	207,657
Amortization of discount and origination fees	12,858
Collection of principal	(860,618)
Balance at September 30, 2018	$ 4,157,653